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Commit a3871bd4 authored by Linus Torvalds's avatar Linus Torvalds
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Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 

Pull RCU updates from Ingo Molnar:
 "The main changes are:

   - Documentation updates, including fixes to the design-level
     requirements documentation and a fixed version of the design-level
     data-structure documentation.  These fixes include removing
     cartoons and getting rid of the html/htmlx duplication.

   - Further improvements to the new-age expedited grace periods.

   - Miscellaneous fixes.

   - Torture-test changes, including a new rcuperf module for measuring
     RCU grace-period performance and scalability, which is useful for
     the expedited-grace-period changes"

* 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (56 commits)
  rcutorture: Add boot-time adjustment of leaf fanout
  rcutorture: Add irqs-disabled test for call_rcu()
  rcutorture: Dump trace buffer upon shutdown
  rcutorture: Don't rebuild identical kernel
  rcutorture: Add OS-jitter capability
  documentation: Add documentation for RCU's major data structures
  rcutorture: Convert test duration to seconds early
  torture: Kill qemu, not parent process
  torture: Clarify refusal to run more than one torture test
  rcutorture: Consider FROZEN hotplug notifier transitions
  rcutorture: Remove redundant initialization to zero
  rcuperf: Do not wake up shutdown wait queue if "shutdown" is false.
  rcutorture: Add largish-system rcuperf scenario
  rcutorture: Avoid RCU CPU stall warning and RT throttling
  rcutorture: Add rcuperf holdoff boot parameter to reduce interference
  rcutorture: Make scripts analyze rcuperf trace data, if present
  rcutorture: Make rcuperf collect expedited event-trace data
  rcutorture: Print measure of batching efficiency
  rcutorture: Set rcuperf writer kthreads to real-time priority
  rcutorture: Bind rcuperf reader/writer kthreads to CPUs
  ...
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<head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
<meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<p>January 27, 2016</p>
<p>This article was contributed by Paul E.&nbsp;McKenney</p>
<h3>Introduction</h3>
This document describes RCU's major data structures and their relationship
to each other.
<ol>
<li> <a href="#Data-Structure Relationships">
Data-Structure Relationships</a>
<li> <a href="#The rcu_state Structure">
The <tt>rcu_state</tt> Structure</a>
<li> <a href="#The rcu_node Structure">
The <tt>rcu_node</tt> Structure</a>
<li> <a href="#The rcu_data Structure">
The <tt>rcu_data</tt> Structure</a>
<li> <a href="#The rcu_dynticks Structure">
The <tt>rcu_dynticks</tt> Structure</a>
<li> <a href="#The rcu_head Structure">
The <tt>rcu_head</tt> Structure</a>
<li> <a href="#RCU-Specific Fields in the task_struct Structure">
RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
<li> <a href="#Accessor Functions">
Accessor Functions</a>
</ol>
At the end we have the
<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
<p>RCU is for all intents and purposes a large state machine, and its
data structures maintain the state in such a way as to allow RCU readers
to execute extremely quickly, while also processing the RCU grace periods
requested by updaters in an efficient and extremely scalable fashion.
The efficiency and scalability of RCU updaters is provided primarily
by a combining tree, as shown below:
</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
containing a tree of <tt>rcu_node</tt> structures.
Each leaf node of the <tt>rcu_node</tt> tree has up to 16
<tt>rcu_data</tt> structures associated with it, so that there
are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
one for each possible CPU.
This structure is adjusted at boot time, if needed, to handle the
common case where <tt>nr_cpu_ids</tt> is much less than
<tt>NR_CPUs</tt>.
For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
which results in a three-level <tt>rcu_node</tt> tree.
If the actual hardware has only 16 CPUs, RCU will adjust itself
at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
</p><p>The purpose of this combining tree is to allow per-CPU events
such as quiescent states, dyntick-idle transitions,
and CPU hotplug operations to be processed efficiently
and scalably.
Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
and other events are recorded by the leaf-level <tt>rcu_node</tt>
structures.
All of these events are combined at each level of the tree until finally
grace periods are completed at the tree's root <tt>rcu_node</tt>
structure.
A grace period can be completed at the root once every CPU
(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task)
has passed through a quiescent state.
Once a grace period has completed, record of that fact is propagated
back down the tree.
</p><p>As can be seen from the diagram, on a 64-bit system
a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
of 64 at the root and a fanout of 16 at the leaves.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why isn't the fanout at the leaves also 64?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Because there are more types of events that affect the leaf-level
<tt>rcu_node</tt> structures than further up the tree.
Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of
64, the contention on these structures' <tt>-&gt;structures</tt>
becomes excessive.
Experimentation on a wide variety of systems has shown that a fanout
of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
</font>
<p><font color="ffffff">Of course, further experience with
systems having hundreds or thousands of CPUs may demonstrate
that the fanout for the non-leaf <tt>rcu_node</tt> structures
must also be reduced.
Such reduction can be easily carried out when and if it proves
necessary.
In the meantime, if you are using such a system and running into
contention problems on the non-leaf <tt>rcu_node</tt> structures,
you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
parameter to reduce the non-leaf fanout as needed.
</font>
<p><font color="ffffff">Kernels built for systems with
strong NUMA characteristics might also need to adjust
<tt>CONFIG_RCU_FANOUT</tt> so that the domains of the
<tt>rcu_node</tt> structures align with hardware boundaries.
However, there has thus far been no need for this.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
a 32-bit system), then RCU will automatically add more levels to the
tree.
For example, if you are crazy enough to build a 64-bit system with 65,536
CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
32-bit systems.
On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
as small as 2 if you wish, which would permit only 16 CPUs, which
is useful for testing.
</p><p>This multi-level combining tree allows us to get most of the
performance and scalability
benefits of partitioning, even though RCU grace-period detection is
inherently a global operation.
The trick here is that only the last CPU to report a quiescent state
into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
structure at the next level up the tree.
This means that at the leaf-level <tt>rcu_node</tt> structure, only
one access out of sixteen will progress up the tree.
For the internal <tt>rcu_node</tt> structures, the situation is even
more extreme: Only one access out of sixty-four will progress up
the tree.
Because the vast majority of the CPUs do not progress up the tree,
the lock contention remains roughly constant up the tree.
No matter how many CPUs there are in the system, at most 64 quiescent-state
reports per grace period will progress all the way to the root
<tt>rcu_node</tt> structure, thus ensuring that the lock contention
on that root <tt>rcu_node</tt> structure remains acceptably low.
</p><p>In effect, the combining tree acts like a big shock absorber,
keeping lock contention under control at all tree levels regardless
of the level of loading on the system.
</p><p>The Linux kernel actually supports multiple flavors of RCU
running concurrently, so RCU builds separate data structures for each
flavor.
For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides
rcu_sched and rcu_bh, as shown below:
</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%">
</p><p>Energy efficiency is increasingly important, and for that
reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which
turns off the scheduling-clock interrupts on idle CPUs, which in
turn allows those CPUs to attain deeper sleep states and to consume
less energy.
CPUs whose scheduling-clock interrupts have been turned off are
said to be in <i>dyntick-idle mode</i>.
RCU must handle dyntick-idle CPUs specially
because RCU would otherwise wake up each CPU on every grace period,
which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>.
RCU uses the <tt>rcu_dynticks</tt> structure to track
which CPUs are in dyntick idle mode, as shown below:
</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%">
</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode
for all flavors of RCU.
Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per
CPU, and all of a given CPU's <tt>rcu_data</tt> structures share
that <tt>rcu_dynticks</tt>, as shown in the figure.
</p><p>Kernels built with <tt>CONFIG_PREEMPT_RCU</tt> support
rcu_preempt in addition to rcu_sched and rcu_bh, as shown below:
</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%">
</p><p>RCU updaters wait for normal grace periods by registering
RCU callbacks, either directly via <tt>call_rcu()</tt> and
friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
there being a separate interface per flavor of RCU)
or indirectly via <tt>synchronize_rcu()</tt> and friends.
RCU callbacks are represented by <tt>rcu_head</tt> structures,
which are queued on <tt>rcu_data</tt> structures while they are
waiting for a grace period to elapse, as shown in the following figure:
</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
</p><p>This figure shows how <tt>TREE_RCU</tt>'s and
<tt>PREEMPT_RCU</tt>'s major data structures are related.
Lesser data structures will be introduced with the algorithms that
make use of them.
</p><p>Note that each of the data structures in the above figure has
its own synchronization:
<p><ol>
<li> Each <tt>rcu_state</tt> structures has a lock and a mutex,
and some fields are protected by the corresponding root
<tt>rcu_node</tt> structure's lock.
<li> Each <tt>rcu_node</tt> structure has a spinlock.
<li> The fields in <tt>rcu_data</tt> are private to the corresponding
CPU, although a few can be read and written by other CPUs.
<li> Similarly, the fields in <tt>rcu_dynticks</tt> are private
to the corresponding CPU, although a few can be read by
other CPUs.
</ol>
<p>It is important to note that different data structures can have
very different ideas about the state of RCU at any given time.
For but one example, awareness of the start or end of a given RCU
grace period propagates slowly through the data structures.
This slow propagation is absolutely necessary for RCU to have good
read-side performance.
If this balkanized implementation seems foreign to you, one useful
trick is to consider each instance of these data structures to be
a different person, each having the usual slightly different
view of reality.
</p><p>The general role of each of these data structures is as
follows:
</p><ol>
<li> <tt>rcu_state</tt>:
This structure forms the interconnection between the
<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
tracks grace periods, serves as short-term repository
for callbacks orphaned by CPU-hotplug events,
maintains <tt>rcu_barrier()</tt> state,
tracks expedited grace-period state,
and maintains state used to force quiescent states when
grace periods extend too long,
<li> <tt>rcu_node</tt>: This structure forms the combining
tree that propagates quiescent-state
information from the leaves to the root, and also propagates
grace-period information from the root to the leaves.
It provides local copies of the grace-period state in order
to allow this information to be accessed in a synchronized
manner without suffering the scalability limitations that
would otherwise be imposed by global locking.
In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists
of tasks that have blocked while in their current
RCU read-side critical section.
In <tt>CONFIG_PREEMPT_RCU</tt> with
<tt>CONFIG_RCU_BOOST</tt>, it manages the
per-<tt>rcu_node</tt> priority-boosting
kernel threads (kthreads) and state.
Finally, it records CPU-hotplug state in order to determine
which CPUs should be ignored during a given grace period.
<li> <tt>rcu_data</tt>: This per-CPU structure is the
focus of quiescent-state detection and RCU callback queuing.
It also tracks its relationship to the corresponding leaf
<tt>rcu_node</tt> structure to allow more-efficient
propagation of quiescent states up the <tt>rcu_node</tt>
combining tree.
Like the <tt>rcu_node</tt> structure, it provides a local
copy of the grace-period information to allow for-free
synchronized
access to this information from the corresponding CPU.
Finally, this structure records past dyntick-idle state
for the corresponding CPU and also tracks statistics.
<li> <tt>rcu_dynticks</tt>:
This per-CPU structure tracks the current dyntick-idle
state for the corresponding CPU.
Unlike the other three structures, the <tt>rcu_dynticks</tt>
structure is not replicated per RCU flavor.
<li> <tt>rcu_head</tt>:
This structure represents RCU callbacks, and is the
only structure allocated and managed by RCU users.
The <tt>rcu_head</tt> structure is normally embedded
within the RCU-protected data structure.
</ol>
<p>If all you wanted from this article was a general notion of how
RCU's data structures are related, you are done.
Otherwise, each of the following sections give more details on
the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>,
and <tt>rcu_dynticks</tt> data structures.
<h3><a name="The rcu_state Structure">
The <tt>rcu_state</tt> Structure</a></h3>
<p>The <tt>rcu_state</tt> structure is the base structure that
represents a flavor of RCU.
This structure forms the interconnection between the
<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
tracks grace periods, contains the lock used to
synchronize with CPU-hotplug events,
and maintains state used to force quiescent states when
grace periods extend too long,
</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
singly and in groups, in the following sections.
The more specialized fields are covered in the discussion of their
use.
<h5>Relationship to rcu_node and rcu_data Structures</h5>
This portion of the <tt>rcu_state</tt> structure is declared
as follows:
<pre>
1 struct rcu_node node[NUM_RCU_NODES];
2 struct rcu_node *level[NUM_RCU_LVLS + 1];
3 struct rcu_data __percpu *rda;
</pre>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Wait a minute!
You said that the <tt>rcu_node</tt> structures formed a tree,
but they are declared as a flat array!
What gives?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
The tree is laid out in the array.
The first node In the array is the head, the next set of nodes in the
array are children of the head node, and so on until the last set of
nodes in the array are the leaves.
</font>
<p><font color="ffffff">See the following diagrams to see how
this works.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>The <tt>rcu_node</tt> tree is embedded into the
<tt>-&gt;node[]</tt> array as shown in the following figure:
</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
</p><p>One interesting consequence of this mapping is that a
breadth-first traversal of the tree is implemented as a simple
linear scan of the array, which is in fact what the
<tt>rcu_for_each_node_breadth_first()</tt> macro does.
This macro is used at the beginning and ends of grace periods.
</p><p>Each entry of the <tt>-&gt;level</tt> array references
the first <tt>rcu_node</tt> structure on the corresponding level
of the tree, for example, as shown below:
</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
</p><p>The zero<sup>th</sup> element of the array references the root
<tt>rcu_node</tt> structure, the first element references the
first child of the root <tt>rcu_node</tt>, and finally the second
element references the first leaf <tt>rcu_node</tt> structure.
</p><p>For whatever it is worth, if you draw the tree to be tree-shaped
rather than array-shaped, it is easy to draw a planar representation:
</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
</p><p>Finally, the <tt>-&gt;rda</tt> field references a per-CPU
pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
</p><p>All of these fields are constant once initialization is complete,
and therefore need no protection.
<h5>Grace-Period Tracking</h5>
<p>This portion of the <tt>rcu_state</tt> structure is declared
as follows:
<pre>
1 unsigned long gpnum;
2 unsigned long completed;
</pre>
<p>RCU grace periods are numbered, and
the <tt>-&gt;gpnum</tt> field contains the number of the grace
period that started most recently.
The <tt>-&gt;completed</tt> field contains the number of the
grace period that completed most recently.
If the two fields are equal, the RCU grace period that most recently
started has already completed, and therefore the corresponding
flavor of RCU is idle.
If <tt>-&gt;gpnum</tt> is one greater than <tt>-&gt;completed</tt>,
then <tt>-&gt;gpnum</tt> gives the number of the current RCU
grace period, which has not yet completed.
Any other combination of values indicates that something is broken.
These two fields are protected by the root <tt>rcu_node</tt>'s
<tt>-&gt;lock</tt> field.
</p><p>There are <tt>-&gt;gpnum</tt> and <tt>-&gt;completed</tt> fields
in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
as well.
The fields in the <tt>rcu_state</tt> structure represent the
most current values, and those of the other structures are compared
in order to detect the start of a new grace period in a distributed
fashion.
The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
<h5>Miscellaneous</h5>
<p>This portion of the <tt>rcu_state</tt> structure is declared
as follows:
<pre>
1 unsigned long gp_max;
2 char abbr;
3 char *name;
</pre>
<p>The <tt>-&gt;gp_max</tt> field tracks the duration of the longest
grace period in jiffies.
It is protected by the root <tt>rcu_node</tt>'s <tt>-&gt;lock</tt>.
<p>The <tt>-&gt;name</tt> field points to the name of the RCU flavor
(for example, &ldquo;rcu_sched&rdquo;), and is constant.
The <tt>-&gt;abbr</tt> field contains a one-character abbreviation,
for example, &ldquo;s&rdquo; for RCU-sched.
<h3><a name="The rcu_node Structure">
The <tt>rcu_node</tt> Structure</a></h3>
<p>The <tt>rcu_node</tt> structures form the combining
tree that propagates quiescent-state
information from the leaves to the root and also that propagates
grace-period information from the root down to the leaves.
They provides local copies of the grace-period state in order
to allow this information to be accessed in a synchronized
manner without suffering the scalability limitations that
would otherwise be imposed by global locking.
In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists
of tasks that have blocked while in their current
RCU read-side critical section.
In <tt>CONFIG_PREEMPT_RCU</tt> with
<tt>CONFIG_RCU_BOOST</tt>, they manage the
per-<tt>rcu_node</tt> priority-boosting
kernel threads (kthreads) and state.
Finally, they record CPU-hotplug state in order to determine
which CPUs should be ignored during a given grace period.
</p><p>The <tt>rcu_node</tt> structure's fields are discussed,
singly and in groups, in the following sections.
<h5>Connection to Combining Tree</h5>
<p>This portion of the <tt>rcu_node</tt> structure is declared
as follows:
<pre>
1 struct rcu_node *parent;
2 u8 level;
3 u8 grpnum;
4 unsigned long grpmask;
5 int grplo;
6 int grphi;
</pre>
<p>The <tt>-&gt;parent</tt> pointer references the <tt>rcu_node</tt>
one level up in the tree, and is <tt>NULL</tt> for the root
<tt>rcu_node</tt>.
The RCU implementation makes heavy use of this field to push quiescent
states up the tree.
The <tt>-&gt;level</tt> field gives the level in the tree, with
the root being at level zero, its children at level one, and so on.
The <tt>-&gt;grpnum</tt> field gives this node's position within
the children of its parent, so this number can range between 0 and 31
on 32-bit systems and between 0 and 63 on 64-bit systems.
The <tt>-&gt;level</tt> and <tt>-&gt;grpnum</tt> fields are
used only during initialization and for tracing.
The <tt>-&gt;grpmask</tt> field is the bitmask counterpart of
<tt>-&gt;grpnum</tt>, and therefore always has exactly one bit set.
This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
structure in its parent's bitmasks, which are described later.
Finally, the <tt>-&gt;grplo</tt> and <tt>-&gt;grphi</tt> fields
contain the lowest and highest numbered CPU served by this
<tt>rcu_node</tt> structure, respectively.
</p><p>All of these fields are constant, and thus do not require any
synchronization.
<h5>Synchronization</h5>
<p>This field of the <tt>rcu_node</tt> structure is declared
as follows:
<pre>
1 raw_spinlock_t lock;
</pre>
<p>This field is used to protect the remaining fields in this structure,
unless otherwise stated.
That said, all of the fields in this structure can be accessed without
locking for tracing purposes.
Yes, this can result in confusing traces, but better some tracing confusion
than to be heisenbugged out of existence.
<h5>Grace-Period Tracking</h5>
<p>This portion of the <tt>rcu_node</tt> structure is declared
as follows:
<pre>
1 unsigned long gpnum;
2 unsigned long completed;
</pre>
<p>These fields are the counterparts of the fields of the same name in
the <tt>rcu_state</tt> structure.
They each may lag up to one behind their <tt>rcu_state</tt>
counterparts.
If a given <tt>rcu_node</tt> structure's <tt>-&gt;gpnum</tt> and
<tt>-&gt;complete</tt> fields are equal, then this <tt>rcu_node</tt>
structure believes that RCU is idle.
Otherwise, as with the <tt>rcu_state</tt> structure,
the <tt>-&gt;gpnum</tt> field will be one greater than the
<tt>-&gt;complete</tt> fields, with <tt>-&gt;gpnum</tt>
indicating which grace period this <tt>rcu_node</tt> believes
is still being waited for.
</p><p>The <tt>&gt;gpnum</tt> field of each <tt>rcu_node</tt>
structure is updated at the beginning
of each grace period, and the <tt>-&gt;completed</tt> fields are
updated at the end of each grace period.
<h5>Quiescent-State Tracking</h5>
<p>These fields manage the propagation of quiescent states up the
combining tree.
</p><p>This portion of the <tt>rcu_node</tt> structure has fields
as follows:
<pre>
1 unsigned long qsmask;
2 unsigned long expmask;
3 unsigned long qsmaskinit;
4 unsigned long expmaskinit;
</pre>
<p>The <tt>-&gt;qsmask</tt> field tracks which of this
<tt>rcu_node</tt> structure's children still need to report
quiescent states for the current normal grace period.
Such children will have a value of 1 in their corresponding bit.
Note that the leaf <tt>rcu_node</tt> structures should be
thought of as having <tt>rcu_data</tt> structures as their
children.
Similarly, the <tt>-&gt;expmask</tt> field tracks which
of this <tt>rcu_node</tt> structure's children still need to report
quiescent states for the current expedited grace period.
An expedited grace period has
the same conceptual properties as a normal grace period, but the
expedited implementation accepts extreme CPU overhead to obtain
much lower grace-period latency, for example, consuming a few
tens of microseconds worth of CPU time to reduce grace-period
duration from milliseconds to tens of microseconds.
The <tt>-&gt;qsmaskinit</tt> field tracks which of this
<tt>rcu_node</tt> structure's children cover for at least
one online CPU.
This mask is used to initialize <tt>-&gt;qsmask</tt>,
and <tt>-&gt;expmaskinit</tt> is used to initialize
<tt>-&gt;expmask</tt> and the beginning of the
normal and expedited grace periods, respectively.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why are these bitmasks protected by locking?
Come on, haven't you heard of atomic instructions???
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Lockless grace-period computation! Such a tantalizing possibility!
</font>
<p><font color="ffffff">But consider the following sequence of events:
</font>
<ol>
<li> <font color="ffffff">CPU&nbsp;0 has been in dyntick-idle
mode for quite some time.
When it wakes up, it notices that the current RCU
grace period needs it to report in, so it sets a
flag where the scheduling clock interrupt will find it.
</font><p>
<li> <font color="ffffff">Meanwhile, CPU&nbsp;1 is running
<tt>force_quiescent_state()</tt>,
and notices that CPU&nbsp;0 has been in dyntick idle mode,
which qualifies as an extended quiescent state.
</font><p>
<li> <font color="ffffff">CPU&nbsp;0's scheduling clock
interrupt fires in the
middle of an RCU read-side critical section, and notices
that the RCU core needs something, so commences RCU softirq
processing.
</font>
<p>
<li> <font color="ffffff">CPU&nbsp;0's softirq handler
executes and is just about ready
to report its quiescent state up the <tt>rcu_node</tt>
tree.
</font><p>
<li> <font color="ffffff">But CPU&nbsp;1 beats it to the punch,
completing the current
grace period and starting a new one.
</font><p>
<li> <font color="ffffff">CPU&nbsp;0 now reports its quiescent
state for the wrong
grace period.
That grace period might now end before the RCU read-side
critical section.
If that happens, disaster will ensue.
</font>
</ol>
<p><font color="ffffff">So the locking is absolutely required in
order to coordinate
clearing of the bits with the grace-period numbers in
<tt>-&gt;gpnum</tt> and <tt>-&gt;completed</tt>.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h5>Blocked-Task Management</h5>
<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the
midst of their RCU read-side critical sections, and these tasks
must be tracked explicitly.
The details of exactly why and how they are tracked will be covered
in a separate article on RCU read-side processing.
For now, it is enough to know that the <tt>rcu_node</tt>
structure tracks them.
<pre>
1 struct list_head blkd_tasks;
2 struct list_head *gp_tasks;
3 struct list_head *exp_tasks;
4 bool wait_blkd_tasks;
</pre>
<p>The <tt>-&gt;blkd_tasks</tt> field is a list header for
the list of blocked and preempted tasks.
As tasks undergo context switches within RCU read-side critical
sections, their <tt>task_struct</tt> structures are enqueued
(via the <tt>task_struct</tt>'s <tt>-&gt;rcu_node_entry</tt>
field) onto the head of the <tt>-&gt;blkd_tasks</tt> list for the
leaf <tt>rcu_node</tt> structure corresponding to the CPU
on which the outgoing context switch executed.
As these tasks later exit their RCU read-side critical sections,
they remove themselves from the list.
This list is therefore in reverse time order, so that if one of the tasks
is blocking the current grace period, all subsequent tasks must
also be blocking that same grace period.
Therefore, a single pointer into this list suffices to track
all tasks blocking a given grace period.
That pointer is stored in <tt>-&gt;gp_tasks</tt> for normal
grace periods and in <tt>-&gt;exp_tasks</tt> for expedited
grace periods.
These last two fields are <tt>NULL</tt> if either there is
no grace period in flight or if there are no blocked tasks
preventing that grace period from completing.
If either of these two pointers is referencing a task that
removes itself from the <tt>-&gt;blkd_tasks</tt> list,
then that task must advance the pointer to the next task on
the list, or set the pointer to <tt>NULL</tt> if there
are no subsequent tasks on the list.
</p><p>For example, suppose that tasks&nbsp;T1, T2, and&nbsp;T3 are
all hard-affinitied to the largest-numbered CPU in the system.
Then if task&nbsp;T1 blocked in an RCU read-side
critical section, then an expedited grace period started,
then task&nbsp;T2 blocked in an RCU read-side critical section,
then a normal grace period started, and finally task&nbsp;3 blocked
in an RCU read-side critical section, then the state of the
last leaf <tt>rcu_node</tt> structure's blocked-task list
would be as shown below:
</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
</p><p>Task&nbsp;T1 is blocking both grace periods, task&nbsp;T2 is
blocking only the normal grace period, and task&nbsp;T3 is blocking
neither grace period.
Note that these tasks will not remove themselves from this list
immediately upon resuming execution.
They will instead remain on the list until they execute the outermost
<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
section.
<p>
The <tt>-&gt;wait_blkd_tasks</tt> field indicates whether or not
the current grace period is waiting on a blocked task.
<h5>Sizing the <tt>rcu_node</tt> Array</h5>
<p>The <tt>rcu_node</tt> array is sized via a series of
C-preprocessor expressions as follows:
<pre>
1 #ifdef CONFIG_RCU_FANOUT
2 #define RCU_FANOUT CONFIG_RCU_FANOUT
3 #else
4 # ifdef CONFIG_64BIT
5 # define RCU_FANOUT 64
6 # else
7 # define RCU_FANOUT 32
8 # endif
9 #endif
10
11 #ifdef CONFIG_RCU_FANOUT_LEAF
12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
13 #else
14 # ifdef CONFIG_64BIT
15 # define RCU_FANOUT_LEAF 64
16 # else
17 # define RCU_FANOUT_LEAF 32
18 # endif
19 #endif
20
21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF)
22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT)
23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT)
24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT)
25
26 #if NR_CPUS &lt;= RCU_FANOUT_1
27 # define RCU_NUM_LVLS 1
28 # define NUM_RCU_LVL_0 1
29 # define NUM_RCU_NODES NUM_RCU_LVL_0
30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 }
31 # define RCU_NODE_NAME_INIT { "rcu_node_0" }
32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" }
33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" }
34 #elif NR_CPUS &lt;= RCU_FANOUT_2
35 # define RCU_NUM_LVLS 2
36 # define NUM_RCU_LVL_0 1
37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" }
41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" }
42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" }
43 #elif NR_CPUS &lt;= RCU_FANOUT_3
44 # define RCU_NUM_LVLS 3
45 # define NUM_RCU_LVL_0 1
46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
53 #elif NR_CPUS &lt;= RCU_FANOUT_4
54 # define RCU_NUM_LVLS 4
55 # define NUM_RCU_LVL_0 1
56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
64 #else
65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
66 #endif
</pre>
<p>The maximum number of levels in the <tt>rcu_node</tt> structure
is currently limited to four, as specified by lines&nbsp;21-24
and the structure of the subsequent &ldquo;if&rdquo; statement.
For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
should be sufficient for the next few years at least.
For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
should see us through the next decade or so.
This four-level tree also allows kernels built with
<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
which might be useful in very large systems having eight CPUs per
socket (but please note that no one has yet shown any measurable
performance degradation due to misaligned socket and <tt>rcu_node</tt>
boundaries).
In addition, building kernels with a full four levels of <tt>rcu_node</tt>
tree permits better testing of RCU's combining-tree code.
</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
it is set based on the word size of the system, which is also
the Kconfig default.
</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
handled by each leaf <tt>rcu_node</tt> structure.
Experience has shown that allowing a given leaf <tt>rcu_node</tt>
structure to handle 64 CPUs, as permitted by the number of bits in
the <tt>-&gt;qsmask</tt> field on a 64-bit system, results in
excessive contention for the leaf <tt>rcu_node</tt> structures'
<tt>-&gt;lock</tt> fields.
The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
selected is based on the word size of the system, just as for
<tt>CONFIG_RCU_FANOUT</tt>.
Lines&nbsp;11-19 perform this computation.
</p><p>Lines&nbsp;21-24 compute the maximum number of CPUs supported by
a single-level (which contains a single <tt>rcu_node</tt> structure),
two-level, three-level, and four-level <tt>rcu_node</tt> tree,
respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
and <tt>RCU_FANOUT_LEAF</tt>.
These numbers of CPUs are retained in the
<tt>RCU_FANOUT_1</tt>,
<tt>RCU_FANOUT_2</tt>,
<tt>RCU_FANOUT_3</tt>, and
<tt>RCU_FANOUT_4</tt>
C-preprocessor variables, respectively.
</p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
statement spanning lines&nbsp;26-66 that computes the number of
<tt>rcu_node</tt> structures required for each level of the tree,
as well as the number of levels required.
The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
C-preprocessor variable by lines&nbsp;27, 35, 44, and&nbsp;54.
The number of <tt>rcu_node</tt> structures for the topmost level
of the tree is always exactly one, and this value is unconditionally
placed into <tt>NUM_RCU_LVL_0</tt> by lines&nbsp;28, 36, 45, and&nbsp;55.
The rest of the levels (if any) of the <tt>rcu_node</tt> tree
are computed by dividing the maximum number of CPUs by the
fanout supported by the number of levels from the current level down,
rounding up. This computation is performed by lines&nbsp;37,
46-47, and&nbsp;56-58.
Lines&nbsp;31-33, 40-42, 50-52, and&nbsp;62-63 create initializers
for lockdep lock-class names.
Finally, lines&nbsp;64-66 produce an error if the maximum number of
CPUs is too large for the specified fanout.
<h3><a name="The rcu_data Structure">
The <tt>rcu_data</tt> Structure</a></h3>
<p>The <tt>rcu_data</tt> maintains the per-CPU state for the
corresponding flavor of RCU.
The fields in this structure may be accessed only from the corresponding
CPU (and from tracing) unless otherwise stated.
This structure is the
focus of quiescent-state detection and RCU callback queuing.
It also tracks its relationship to the corresponding leaf
<tt>rcu_node</tt> structure to allow more-efficient
propagation of quiescent states up the <tt>rcu_node</tt>
combining tree.
Like the <tt>rcu_node</tt> structure, it provides a local
copy of the grace-period information to allow for-free
synchronized
access to this information from the corresponding CPU.
Finally, this structure records past dyntick-idle state
for the corresponding CPU and also tracks statistics.
</p><p>The <tt>rcu_data</tt> structure's fields are discussed,
singly and in groups, in the following sections.
<h5>Connection to Other Data Structures</h5>
<p>This portion of the <tt>rcu_data</tt> structure is declared
as follows:
<pre>
1 int cpu;
2 struct rcu_state *rsp;
3 struct rcu_node *mynode;
4 struct rcu_dynticks *dynticks;
5 unsigned long grpmask;
6 bool beenonline;
</pre>
<p>The <tt>-&gt;cpu</tt> field contains the number of the
corresponding CPU, the <tt>-&gt;rsp</tt> pointer references
the corresponding <tt>rcu_state</tt> structure (and is most frequently
used to locate the name of the corresponding flavor of RCU for tracing),
and the <tt>-&gt;mynode</tt> field references the corresponding
<tt>rcu_node</tt> structure.
The <tt>-&gt;mynode</tt> is used to propagate quiescent states
up the combining tree.
<p>The <tt>-&gt;dynticks</tt> pointer references the
<tt>rcu_dynticks</tt> structure corresponding to this
CPU.
Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt>
structure is shared among all flavors of RCU.
These first four fields are constant and therefore require not
synchronization.
</p><p>The <tt>-&gt;grpmask</tt> field indicates the bit in
the <tt>-&gt;mynode-&gt;qsmask</tt> corresponding to this
<tt>rcu_data</tt> structure, and is also used when propagating
quiescent states.
The <tt>-&gt;beenonline</tt> flag is set whenever the corresponding
CPU comes online, which means that the debugfs tracing need not dump
out any <tt>rcu_data</tt> structure for which this flag is not set.
<h5>Quiescent-State and Grace-Period Tracking</h5>
<p>This portion of the <tt>rcu_data</tt> structure is declared
as follows:
<pre>
1 unsigned long completed;
2 unsigned long gpnum;
3 bool cpu_no_qs;
4 bool core_needs_qs;
5 bool gpwrap;
6 unsigned long rcu_qs_ctr_snap;
</pre>
<p>The <tt>completed</tt> and <tt>gpnum</tt>
fields are the counterparts of the fields of the same name
in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
They may each lag up to one behind their <tt>rcu_node</tt>
counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
will catch up upon exit from dyntick-idle mode).
If a given <tt>rcu_data</tt> structure's <tt>-&gt;gpnum</tt> and
<tt>-&gt;complete</tt> fields are equal, then this <tt>rcu_data</tt>
structure believes that RCU is idle.
Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt>
structure,
the <tt>-&gt;gpnum</tt> field will be one greater than the
<tt>-&gt;complete</tt> fields, with <tt>-&gt;gpnum</tt>
indicating which grace period this <tt>rcu_data</tt> believes
is still being waited for.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
All this replication of the grace period numbers can only cause
massive confusion.
Why not just keep a global pair of counters and be done with it???
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Because if there was only a single global pair of grace-period
numbers, there would need to be a single global lock to allow
safely accessing and updating them.
And if we are not going to have a single global lock, we need
to carefully manage the numbers on a per-node basis.
Recall from the answer to a previous Quick Quiz that the consequences
of applying a previously sampled quiescent state to the wrong
grace period are quite severe.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>The <tt>-&gt;cpu_no_qs</tt> flag indicates that the
CPU has not yet passed through a quiescent state,
while the <tt>-&gt;core_needs_qs</tt> flag indicates that the
RCU core needs a quiescent state from the corresponding CPU.
The <tt>-&gt;gpwrap</tt> field indicates that the corresponding
CPU has remained idle for so long that the <tt>completed</tt>
and <tt>gpnum</tt> counters are in danger of overflow, which
will cause the CPU to disregard the values of its counters on
its next exit from idle.
Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
cases where a given operation has resulted in a quiescent state
for all flavors of RCU, for example, <tt>cond_resched_rcu_qs()</tt>.
<h5>RCU Callback Handling</h5>
<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
the same CPU that registered them.
This is strictly a cache-locality optimization: callbacks can and
do get invoked on CPUs other than the one that registered them.
After all, if the CPU that registered a given callback has gone
offline before the callback can be invoked, there really is no other
choice.
</p><p>This portion of the <tt>rcu_data</tt> structure is declared
as follows:
<pre>
1 struct rcu_head *nxtlist;
2 struct rcu_head **nxttail[RCU_NEXT_SIZE];
3 unsigned long nxtcompleted[RCU_NEXT_SIZE];
4 long qlen_lazy;
5 long qlen;
6 long qlen_last_fqs_check;
7 unsigned long n_force_qs_snap;
8 unsigned long n_cbs_invoked;
9 unsigned long n_cbs_orphaned;
10 unsigned long n_cbs_adopted;
11 long blimit;
</pre>
<p>The <tt>-&gt;nxtlist</tt> pointer and the
<tt>-&gt;nxttail[]</tt> array form a four-segment list with
older callbacks near the head and newer ones near the tail.
Each segment contains callbacks with the corresponding relationship
to the current grace period.
The pointer out of the end of each of the four segments is referenced
by the element of the <tt>-&gt;nxttail[]</tt> array indexed by
<tt>RCU_DONE_TAIL</tt> (for callbacks handled by a prior grace period),
<tt>RCU_WAIT_TAIL</tt> (for callbacks waiting on the current grace period),
<tt>RCU_NEXT_READY_TAIL</tt> (for callbacks that will wait on the next
grace period), and
<tt>RCU_NEXT_TAIL</tt> (for callbacks that are not yet associated
with a specific grace period)
respectively, as shown in the following figure.
</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
</p><p>In this figure, the <tt>-&gt;nxtlist</tt> pointer references the
first
RCU callback in the list.
The <tt>-&gt;nxttail[RCU_DONE_TAIL]</tt> array element references
the <tt>-&gt;nxtlist</tt> pointer itself, indicating that none
of the callbacks is ready to invoke.
The <tt>-&gt;nxttail[RCU_WAIT_TAIL]</tt> array element references callback
CB&nbsp;2's <tt>-&gt;next</tt> pointer, which indicates that
CB&nbsp;1 and CB&nbsp;2 are both waiting on the current grace period.
The <tt>-&gt;nxttail[RCU_NEXT_READY_TAIL]</tt> array element
references the same RCU callback that <tt>-&gt;nxttail[RCU_WAIT_TAIL]</tt>
does, which indicates that there are no callbacks waiting on the next
RCU grace period.
The <tt>-&gt;nxttail[RCU_NEXT_TAIL]</tt> array element references
CB&nbsp;4's <tt>-&gt;next</tt> pointer, indicating that all the
remaining RCU callbacks have not yet been assigned to an RCU grace
period.
Note that the <tt>-&gt;nxttail[RCU_NEXT_TAIL]</tt> array element
always references the last RCU callback's <tt>-&gt;next</tt> pointer
unless the callback list is empty, in which case it references
the <tt>-&gt;nxtlist</tt> pointer.
</p><p>CPUs advance their callbacks from the
<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
as grace periods advance.
The CPU advances the callbacks in its <tt>rcu_data</tt> structure
whenever it notices that another RCU grace period has completed.
The CPU detects the completion of an RCU grace period by noticing
that the value of its <tt>rcu_data</tt> structure's
<tt>-&gt;completed</tt> field differs from that of its leaf
<tt>rcu_node</tt> structure.
Recall that each <tt>rcu_node</tt> structure's
<tt>-&gt;completed</tt> field is updated at the end of each
grace period.
</p><p>The <tt>-&gt;nxtcompleted[]</tt> array records grace-period
numbers corresponding to the list segments.
This allows CPUs that go idle for extended periods to determine
which of their callbacks are ready to be invoked after reawakening.
</p><p>The <tt>-&gt;qlen</tt> counter contains the number of
callbacks in <tt>-&gt;nxtlist</tt>, and the
<tt>-&gt;qlen_lazy</tt> contains the number of those callbacks that
are known to only free memory, and whose invocation can therefore
be safely deferred.
The <tt>-&gt;qlen_last_fqs_check</tt> and
<tt>-&gt;n_force_qs_snap</tt> coordinate the forcing of quiescent
states from <tt>call_rcu()</tt> and friends when callback
lists grow excessively long.
</p><p>The <tt>-&gt;n_cbs_invoked</tt>,
<tt>-&gt;n_cbs_orphaned</tt>, and <tt>-&gt;n_cbs_adopted</tt>
fields count the number of callbacks invoked,
sent to other CPUs when this CPU goes offline,
and received from other CPUs when those other CPUs go offline.
Finally, the <tt>-&gt;blimit</tt> counter is the maximum number of
RCU callbacks that may be invoked at a given time.
<h5>Dyntick-Idle Handling</h5>
<p>This portion of the <tt>rcu_data</tt> structure is declared
as follows:
<pre>
1 int dynticks_snap;
2 unsigned long dynticks_fqs;
</pre>
The <tt>-&gt;dynticks_snap</tt> field is used to take a snapshot
of the corresponding CPU's dyntick-idle state when forcing
quiescent states, and is therefore accessed from other CPUs.
Finally, the <tt>-&gt;dynticks_fqs</tt> field is used to
count the number of times this CPU is determined to be in
dyntick-idle state, and is used for tracing and debugging purposes.
<h3><a name="The rcu_dynticks Structure">
The <tt>rcu_dynticks</tt> Structure</a></h3>
<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state
for the corresponding CPU.
Unlike the other structures, <tt>rcu_dynticks</tt> is not
replicated over the different flavors of RCU.
The fields in this structure may be accessed only from the corresponding
CPU (and from tracing) unless otherwise stated.
Its fields are as follows:
<pre>
1 int dynticks_nesting;
2 int dynticks_nmi_nesting;
3 atomic_t dynticks;
</pre>
<p>The <tt>-&gt;dynticks_nesting</tt> field counts the
nesting depth of normal interrupts.
In addition, this counter is incremented when exiting dyntick-idle
mode and decremented when entering it.
This counter can therefore be thought of as counting the number
of reasons why this CPU cannot be permitted to enter dyntick-idle
mode, aside from non-maskable interrupts (NMIs).
NMIs are counted by the <tt>-&gt;dynticks_nmi_nesting</tt>
field, except that NMIs that interrupt non-dyntick-idle execution
are not counted.
</p><p>Finally, the <tt>-&gt;dynticks</tt> field counts the corresponding
CPU's transitions to and from dyntick-idle mode, so that this counter
has an even value when the CPU is in dyntick-idle mode and an odd
value otherwise.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why not just count all NMIs?
Wouldn't that be simpler and less error prone?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
It seems simpler only until you think hard about how to go about
updating the <tt>rcu_dynticks</tt> structure's
<tt>-&gt;dynticks</tt> field.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>Additional fields are present for some special-purpose
builds, and are discussed separately.
<h3><a name="The rcu_head Structure">
The <tt>rcu_head</tt> Structure</a></h3>
<p>Each <tt>rcu_head</tt> structure represents an RCU callback.
These structures are normally embedded within RCU-protected data
structures whose algorithms use asynchronous grace periods.
In contrast, when using algorithms that block waiting for RCU grace periods,
RCU users need not provide <tt>rcu_head</tt> structures.
</p><p>The <tt>rcu_head</tt> structure has fields as follows:
<pre>
1 struct rcu_head *next;
2 void (*func)(struct rcu_head *head);
</pre>
<p>The <tt>-&gt;next</tt> field is used
to link the <tt>rcu_head</tt> structures together in the
lists within the <tt>rcu_data</tt> structures.
The <tt>-&gt;func</tt> field is a pointer to the function
to be called when the callback is ready to be invoked, and
this function is passed a pointer to the <tt>rcu_head</tt>
structure.
However, <tt>kfree_rcu()</tt> uses the <tt>-&gt;func</tt>
field to record the offset of the <tt>rcu_head</tt>
structure within the enclosing RCU-protected data structure.
</p><p>Both of these fields are used internally by RCU.
From the viewpoint of RCU users, this structure is an
opaque &ldquo;cookie&rdquo;.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Given that the callback function <tt>-&gt;func</tt>
is passed a pointer to the <tt>rcu_head</tt> structure,
how is that function supposed to find the beginning of the
enclosing RCU-protected data structure?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In actual practice, there is a separate callback function per
type of RCU-protected data structure.
The callback function can therefore use the <tt>container_of()</tt>
macro in the Linux kernel (or other pointer-manipulation facilities
in other software environments) to find the beginning of the
enclosing structure.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h3><a name="RCU-Specific Fields in the task_struct Structure">
RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
<p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some
additional fields in the <tt>task_struct</tt> structure:
<pre>
1 #ifdef CONFIG_PREEMPT_RCU
2 int rcu_read_lock_nesting;
3 union rcu_special rcu_read_unlock_special;
4 struct list_head rcu_node_entry;
5 struct rcu_node *rcu_blocked_node;
6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
7 #ifdef CONFIG_TASKS_RCU
8 unsigned long rcu_tasks_nvcsw;
9 bool rcu_tasks_holdout;
10 struct list_head rcu_tasks_holdout_list;
11 int rcu_tasks_idle_cpu;
12 #endif /* #ifdef CONFIG_TASKS_RCU */
</pre>
<p>The <tt>-&gt;rcu_read_lock_nesting</tt> field records the
nesting level for RCU read-side critical sections, and
the <tt>-&gt;rcu_read_unlock_special</tt> field is a bitmask
that records special conditions that require <tt>rcu_read_unlock()</tt>
to do additional work.
The <tt>-&gt;rcu_node_entry</tt> field is used to form lists of
tasks that have blocked within preemptible-RCU read-side critical
sections and the <tt>-&gt;rcu_blocked_node</tt> field references
the <tt>rcu_node</tt> structure whose list this task is a member of,
or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
read-side critical section.
<p>The <tt>-&gt;rcu_tasks_nvcsw</tt> field tracks the number of
voluntary context switches that this task had undergone at the
beginning of the current tasks-RCU grace period,
<tt>-&gt;rcu_tasks_holdout</tt> is set if the current tasks-RCU
grace period is waiting on this task, <tt>-&gt;rcu_tasks_holdout_list</tt>
is a list element enqueuing this task on the holdout list,
and <tt>-&gt;rcu_tasks_idle_cpu</tt> tracks which CPU this
idle task is running, but only if the task is currently running,
that is, if the CPU is currently idle.
<h3><a name="Accessor Functions">
Accessor Functions</a></h3>
<p>The following listing shows the
<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>,
<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and
<tt>rcu_for_each_leaf_node()</tt> function and macros:
<pre>
1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
2 {
3 return &amp;rsp-&gt;node[0];
4 }
5
6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
7 for ((rnp) = &amp;(rsp)-&gt;node[0]; \
8 (rnp) &lt; &amp;(rsp)-&gt;node[NUM_RCU_NODES]; (rnp)++)
9
10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
11 for ((rnp) = &amp;(rsp)-&gt;node[0]; \
12 (rnp) &lt; (rsp)-&gt;level[NUM_RCU_LVLS - 1]; (rnp)++)
13
14 #define rcu_for_each_leaf_node(rsp, rnp) \
15 for ((rnp) = (rsp)-&gt;level[NUM_RCU_LVLS - 1]; \
16 (rnp) &lt; &amp;(rsp)-&gt;node[NUM_RCU_NODES]; (rnp)++)
</pre>
<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
first element of the specified <tt>rcu_state</tt> structure's
<tt>-&gt;node[]</tt> array, which is the root <tt>rcu_node</tt>
structure.
</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
macro takes advantage of the layout of the <tt>rcu_node</tt>
structures in the <tt>rcu_state</tt> structure's
<tt>-&gt;node[]</tt> array, performing a breadth-first traversal by
simply traversing the array in order.
The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates
similarly, but traverses only the first part of the array, thus excluding
the leaf <tt>rcu_node</tt> structures.
Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
the last part of the array, thus traversing only the leaf
<tt>rcu_node</tt> structures.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
contains only a single node?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In the single-node case,
<tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op
and <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h3><a name="Summary">
Summary</a></h3>
So each flavor of RCU is represented by an <tt>rcu_state</tt> structure,
which contains a combining tree of <tt>rcu_node</tt> and
<tt>rcu_data</tt> structures.
Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
state is tracked by an <tt>rcu_dynticks</tt> structure.
If you made it this far, you are well prepared to read the code
walkthroughs in the other articles in this series.
<h3><a name="Acknowledgments">
Acknowledgments</a></h3>
I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
for helping me get this document into a more human-readable state.
<h3><a name="Legal Statement">
Legal Statement</a></h3>
<p>This work represents the view of the author and does not necessarily
represent the view of IBM.
</p><p>Linux is a registered trademark of Linus Torvalds.
</p><p>Other company, product, and service names may be trademarks or
service marks of others.
</body></html>
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id="tspan3017">Read-Mostly, Stale &amp;</tspan></text>
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id="tspan3019">Inconsistent Data OK</tspan></text>
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id="tspan3021">(RCU Works Great!!!)</tspan></text>
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id="tspan3023">(RCU Works Well)</tspan></text>
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id="tspan3025">Read-Mostly, Need Consistent Data</tspan></text>
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sodipodi:linespacing="125%"
style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
id="tspan3027">Read-Write, Need Consistent Data</tspan></text>
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<text
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id="text26"
style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
sodipodi:linespacing="125%">Update-Mostly, Need Consistent Data</text>
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<text
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id="text28"
sodipodi:linespacing="125%"
style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
id="tspan3029">(RCU Might Be OK...)</tspan></text>
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<text
xml:space="preserve"
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y="7875"
font-style="normal"
font-weight="normal"
font-size="324"
id="text30"
style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
sodipodi:linespacing="125%">(1) Provide Existence Guarantees For Update-Friendly Mechanisms</text>
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<text
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font-size="324"
id="text32"
style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
sodipodi:linespacing="125%">(2) Provide Wait-Free Read-Side Primitives for Real-Time Use)</text>
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<text
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id="text34"
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sodipodi:linespacing="125%">(RCU is Very Unlikely to be the Right Tool For The Job, But it Can:</text>
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Documentation/RCU/Design/Requirements/Requirements.html
View file @ a3871bd4
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
......@@ -65,8 +63,8 @@ All that aside, here are the categories of currently known RCU requirements:
<p>
This is followed by a <a href="#Summary">summary</a>,
which is in turn followed by the inevitable
<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
however, the answers to each quick quiz immediately follows the quiz.
Select the big white space with your mouse to see the answer.
<h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2>
......@@ -153,13 +151,27 @@ Therefore, the outcome:
</blockquote>
cannot happen.
<p><a name="Quick Quiz 1"><b>Quick Quiz 1</b>:</a>
Wait a minute!
You said that updaters can make useful forward progress concurrently
with readers, but pre-existing readers will block
<tt>synchronize_rcu()</tt>!!!
Just who are you trying to fool???
<br><a href="#qq1answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Wait a minute!
You said that updaters can make useful forward progress concurrently
with readers, but pre-existing readers will block
<tt>synchronize_rcu()</tt>!!!
Just who are you trying to fool???
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
First, if updaters do not wish to be blocked by readers, they can use
<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
be discussed later.
Second, even when using <tt>synchronize_rcu()</tt>, the other
update-side code does run concurrently with readers, whether
pre-existing or not.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
This scenario resembles one of the first uses of RCU in
......@@ -210,9 +222,20 @@ to guarantee that <tt>do_something()</tt> never runs concurrently
with <tt>recovery()</tt>, but with little or no synchronization
overhead in <tt>do_something_dlm()</tt>.
<p><a name="Quick Quiz 2"><b>Quick Quiz 2</b>:</a>
Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
<br><a href="#qq2answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Without that extra grace period, memory reordering could result in
<tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
concurrently with the last bits of <tt>recovery()</tt>.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
In order to avoid fatal problems such as deadlocks,
......@@ -332,12 +355,27 @@ It also prevents any number of &ldquo;interesting&rdquo; compiler
optimizations, for example, the use of <tt>gp</tt> as a scratch
location immediately preceding the assignment.
<p><a name="Quick Quiz 3"><b>Quick Quiz 3</b>:</a>
But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
from being reordered.
Can't that also cause problems?
<br><a href="#qq3answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
from being reordered.
Can't that also cause problems?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
No, it cannot.
The readers cannot see either of these two fields until
the assignment to <tt>gp</tt>, by which time both fields are
fully initialized.
So reordering the assignments
to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt> cannot possibly
cause any problems.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
It is tempting to assume that the reader need not do anything special
......@@ -494,11 +532,42 @@ The <tt>rcu_access_pointer()</tt> on line&nbsp;6 is similar to
code protected by the corresponding update-side lock.
</ol>
<p><a name="Quick Quiz 4"><b>Quick Quiz 4</b>:</a>
Without the <tt>rcu_dereference()</tt> or the
<tt>rcu_access_pointer()</tt>, what destructive optimizations
might the compiler make use of?
<br><a href="#qq4answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Without the <tt>rcu_dereference()</tt> or the
<tt>rcu_access_pointer()</tt>, what destructive optimizations
might the compiler make use of?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Let's start with what happens to <tt>do_something_gp()</tt>
if it fails to use <tt>rcu_dereference()</tt>.
It could reuse a value formerly fetched from this same pointer.
It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
mash-up of two distince pointer values.
It might even use value-speculation optimizations, where it makes
a wrong guess, but by the time it gets around to checking the
value, an update has changed the pointer to match the wrong guess.
Too bad about any dereferences that returned pre-initialization garbage
in the meantime!
</font>
<p><font color="ffffff">
For <tt>remove_gp_synchronous()</tt>, as long as all modifications
to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
the above optimizations are harmless.
However,
with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>,
<tt>sparse</tt> will complain if you
define <tt>gp</tt> with <tt>__rcu</tt> and then
access it without using
either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
In short, RCU's publish-subscribe guarantee is provided by the combination
......@@ -571,17 +640,156 @@ systems with more than one CPU:
<tt>synchronize_rcu()</tt> migrates in the meantime.
</ol>
<p><a name="Quick Quiz 5"><b>Quick Quiz 5</b>:</a>
Given that multiple CPUs can start RCU read-side critical sections
at any time without any ordering whatsoever, how can RCU possibly tell whether
or not a given RCU read-side critical section starts before a
given instance of <tt>synchronize_rcu()</tt>?
<br><a href="#qq5answer">Answer</a>
<p><a name="Quick Quiz 6"><b>Quick Quiz 6</b>:</a>
The first and second guarantees require unbelievably strict ordering!
Are all these memory barriers <i> really</i> required?
<br><a href="#qq6answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Given that multiple CPUs can start RCU read-side critical sections
at any time without any ordering whatsoever, how can RCU possibly
tell whether or not a given RCU read-side critical section starts
before a given instance of <tt>synchronize_rcu()</tt>?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
If RCU cannot tell whether or not a given
RCU read-side critical section starts before a
given instance of <tt>synchronize_rcu()</tt>,
then it must assume that the RCU read-side critical section
started first.
In other words, a given instance of <tt>synchronize_rcu()</tt>
can avoid waiting on a given RCU read-side critical section only
if it can prove that <tt>synchronize_rcu()</tt> started first.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
The first and second guarantees require unbelievably strict ordering!
Are all these memory barriers <i> really</i> required?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Yes, they really are required.
To see why the first guarantee is required, consider the following
sequence of events:
</font>
<ol>
<li> <font color="ffffff">
CPU 1: <tt>rcu_read_lock()</tt>
</font>
<li> <font color="ffffff">
CPU 1: <tt>q = rcu_dereference(gp);
/* Very likely to return p. */</tt>
</font>
<li> <font color="ffffff">
CPU 0: <tt>list_del_rcu(p);</tt>
</font>
<li> <font color="ffffff">
CPU 0: <tt>synchronize_rcu()</tt> starts.
</font>
<li> <font color="ffffff">
CPU 1: <tt>do_something_with(q-&gt;a);
/* No smp_mb(), so might happen after kfree(). */</tt>
</font>
<li> <font color="ffffff">
CPU 1: <tt>rcu_read_unlock()</tt>
</font>
<li> <font color="ffffff">
CPU 0: <tt>synchronize_rcu()</tt> returns.
</font>
<li> <font color="ffffff">
CPU 0: <tt>kfree(p);</tt>
</font>
</ol>
<p><font color="ffffff">
Therefore, there absolutely must be a full memory barrier between the
end of the RCU read-side critical section and the end of the
grace period.
</font>
<p><font color="ffffff">
The sequence of events demonstrating the necessity of the second rule
is roughly similar:
</font>
<ol>
<li> <font color="ffffff">CPU 0: <tt>list_del_rcu(p);</tt>
</font>
<li> <font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> starts.
</font>
<li> <font color="ffffff">CPU 1: <tt>rcu_read_lock()</tt>
</font>
<li> <font color="ffffff">CPU 1: <tt>q = rcu_dereference(gp);
/* Might return p if no memory barrier. */</tt>
</font>
<li> <font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> returns.
</font>
<li> <font color="ffffff">CPU 0: <tt>kfree(p);</tt>
</font>
<li> <font color="ffffff">
CPU 1: <tt>do_something_with(q-&gt;a); /* Boom!!! */</tt>
</font>
<li> <font color="ffffff">CPU 1: <tt>rcu_read_unlock()</tt>
</font>
</ol>
<p><font color="ffffff">
And similarly, without a memory barrier between the beginning of the
grace period and the beginning of the RCU read-side critical section,
CPU&nbsp;1 might end up accessing the freelist.
</font>
<p><font color="ffffff">
The &ldquo;as if&rdquo; rule of course applies, so that any
implementation that acts as if the appropriate memory barriers
were in place is a correct implementation.
That said, it is much easier to fool yourself into believing
that you have adhered to the as-if rule than it is to actually
adhere to it!
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
You claim that <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
generate absolutely no code in some kernel builds.
This means that the compiler might arbitrarily rearrange consecutive
RCU read-side critical sections.
Given such rearrangement, if a given RCU read-side critical section
is done, how can you be sure that all prior RCU read-side critical
sections are done?
Won't the compiler rearrangements make that impossible to determine?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In cases where <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
generate absolutely no code, RCU infers quiescent states only at
special locations, for example, within the scheduler.
Because calls to <tt>schedule()</tt> had better prevent calling-code
accesses to shared variables from being rearranged across the call to
<tt>schedule()</tt>, if RCU detects the end of a given RCU read-side
critical section, it will necessarily detect the end of all prior
RCU read-side critical sections, no matter how aggressively the
compiler scrambles the code.
</font>
<p><font color="ffffff">
Again, this all assumes that the compiler cannot scramble code across
calls to the scheduler, out of interrupt handlers, into the idle loop,
into user-mode code, and so on.
But if your kernel build allows that sort of scrambling, you have broken
far more than just RCU!
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
Note that these memory-barrier requirements do not replace the fundamental
......@@ -626,9 +834,19 @@ inconvenience can be avoided through use of the
<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members
described later in this document.
<p><a name="Quick Quiz 7"><b>Quick Quiz 7</b>:</a>
But how does the upgrade-to-write operation exclude other readers?
<br><a href="#qq7answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But how does the upgrade-to-write operation exclude other readers?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
It doesn't, just like normal RCU updates, which also do not exclude
RCU readers.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
This guarantee allows lookup code to be shared between read-side
......@@ -714,9 +932,20 @@ to do significant reordering.
This is by design: Any significant ordering constraints would slow down
these fast-path APIs.
<p><a name="Quick Quiz 8"><b>Quick Quiz 8</b>:</a>
Can't the compiler also reorder this code?
<br><a href="#qq8answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Can't the compiler also reorder this code?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
No, the volatile casts in <tt>READ_ONCE()</tt> and
<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
this particular case.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3>
......@@ -769,10 +998,28 @@ new readers can start immediately after <tt>synchronize_rcu()</tt>
starts, and <tt>synchronize_rcu()</tt> is under no
obligation to wait for these new readers.
<p><a name="Quick Quiz 9"><b>Quick Quiz 9</b>:</a>
Suppose that synchronize_rcu() did wait until all readers had completed.
Would the updater be able to rely on this?
<br><a href="#qq9answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Suppose that synchronize_rcu() did wait until <i>all</i>
readers had completed instead of waiting only on
pre-existing readers.
For how long would the updater be able to rely on there
being no readers?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
For no time at all.
Even if <tt>synchronize_rcu()</tt> were to wait until
all readers had completed, a new reader might start immediately after
<tt>synchronize_rcu()</tt> completed.
Therefore, the code following
<tt>synchronize_rcu()</tt> can <i>never</i> rely on there being
no readers.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h3><a name="Grace Periods Don't Partition Read-Side Critical Sections">
Grace Periods Don't Partition Read-Side Critical Sections</a></h3>
......@@ -969,11 +1216,24 @@ grace period.
As a result, an RCU read-side critical section cannot partition a pair
of RCU grace periods.
<p><a name="Quick Quiz 10"><b>Quick Quiz 10</b>:</a>
How long a sequence of grace periods, each separated by an RCU read-side
critical section, would be required to partition the RCU read-side
critical sections at the beginning and end of the chain?
<br><a href="#qq10answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
How long a sequence of grace periods, each separated by an RCU
read-side critical section, would be required to partition the RCU
read-side critical sections at the beginning and end of the chain?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In theory, an infinite number.
In practice, an unknown number that is sensitive to both implementation
details and timing considerations.
Therefore, even in practice, RCU users must abide by the
theoretical rather than the practical answer.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<h3><a name="Disabling Preemption Does Not Block Grace Periods">
Disabling Preemption Does Not Block Grace Periods</a></h3>
......@@ -1109,12 +1369,27 @@ These classes is covered in the following sections.
<h3><a name="Specialization">Specialization</a></h3>
<p>
RCU is and always has been intended primarily for read-mostly situations, as
illustrated by the following figure.
This means that RCU's read-side primitives are optimized, often at the
RCU is and always has been intended primarily for read-mostly situations,
which means that RCU's read-side primitives are optimized, often at the
expense of its update-side primitives.
Experience thus far is captured by the following list of situations:
<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p>
<ol>
<li> Read-mostly data, where stale and inconsistent data is not
a problem: RCU works great!
<li> Read-mostly data, where data must be consistent:
RCU works well.
<li> Read-write data, where data must be consistent:
RCU <i>might</i> work OK.
Or not.
<li> Write-mostly data, where data must be consistent:
RCU is very unlikely to be the right tool for the job,
with the following exceptions, where RCU can provide:
<ol type=a>
<li> Existence guarantees for update-friendly mechanisms.
<li> Wait-free read-side primitives for real-time use.
</ol>
</ol>
<p>
This focus on read-mostly situations means that RCU must interoperate
......@@ -1127,9 +1402,43 @@ synchronization primitives be legal within RCU read-side critical sections,
including spinlocks, sequence locks, atomic operations, reference
counters, and memory barriers.
<p><a name="Quick Quiz 11"><b>Quick Quiz 11</b>:</a>
What about sleeping locks?
<br><a href="#qq11answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
What about sleeping locks?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
These are forbidden within Linux-kernel RCU read-side critical
sections because it is not legal to place a quiescent state
(in this case, voluntary context switch) within an RCU read-side
critical section.
However, sleeping locks may be used within userspace RCU read-side
critical sections, and also within Linux-kernel sleepable RCU
<a href="#Sleepable RCU"><font color="ffffff">(SRCU)</font></a>
read-side critical sections.
In addition, the -rt patchset turns spinlocks into a
sleeping locks so that the corresponding critical sections
can be preempted, which also means that these sleeplockified
spinlocks (but not other sleeping locks!) may be acquire within
-rt-Linux-kernel RCU read-side critical sections.
</font>
<p><font color="ffffff">
Note that it <i>is</i> legal for a normal RCU read-side
critical section to conditionally acquire a sleeping locks
(as in <tt>mutex_trylock()</tt>), but only as long as it does
not loop indefinitely attempting to conditionally acquire that
sleeping locks.
The key point is that things like <tt>mutex_trylock()</tt>
either return with the mutex held, or return an error indication if
the mutex was not immediately available.
Either way, <tt>mutex_trylock()</tt> returns immediately without
sleeping.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
It often comes as a surprise that many algorithms do not require a
......@@ -1160,10 +1469,7 @@ some period of time, so the exact wait period is a judgment call.
One of our pair of veternarians might wait 30 seconds before pronouncing
the cat dead, while the other might insist on waiting a full minute.
The two veternarians would then disagree on the state of the cat during
the final 30 seconds of the minute following the last heartbeat, as
fancifully illustrated below:
<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p>
the final 30 seconds of the minute following the last heartbeat.
<p>
Interestingly enough, this same situation applies to hardware.
......@@ -1343,7 +1649,8 @@ situations where neither <tt>synchronize_rcu()</tt> nor
<tt>synchronize_rcu_expedited()</tt> would be legal,
including within preempt-disable code, <tt>local_bh_disable()</tt> code,
interrupt-disable code, and interrupt handlers.
However, even <tt>call_rcu()</tt> is illegal within NMI handlers.
However, even <tt>call_rcu()</tt> is illegal within NMI handlers
and from idle and offline CPUs.
The callback function (<tt>remove_gp_cb()</tt> in this case) will be
executed within softirq (software interrupt) environment within the
Linux kernel,
......@@ -1354,12 +1661,27 @@ write an RCU callback function that takes too long.
Long-running operations should be relegated to separate threads or
(in the Linux kernel) workqueues.
<p><a name="Quick Quiz 12"><b>Quick Quiz 12</b>:</a>
Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
structure, which would interact badly with concurrent insertions.
Doesn't this mean that <tt>rcu_dereference()</tt> is required?
<br><a href="#qq12answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
structure, which would interact badly with concurrent insertions.
Doesn't this mean that <tt>rcu_dereference()</tt> is required?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Presumably the <tt>-&gt;gp_lock</tt> acquired on line&nbsp;18 excludes
any changes, including any insertions that <tt>rcu_dereference()</tt>
would protect against.
Therefore, any insertions will be delayed until after
<tt>-&gt;gp_lock</tt>
is released on line&nbsp;25, which in turn means that
<tt>rcu_access_pointer()</tt> suffices.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
However, all that <tt>remove_gp_cb()</tt> is doing is
......@@ -1406,14 +1728,31 @@ This was due to the fact that RCU was not heavily used within DYNIX/ptx,
so the very few places that needed something like
<tt>synchronize_rcu()</tt> simply open-coded it.
<p><a name="Quick Quiz 13"><b>Quick Quiz 13</b>:</a>
Earlier it was claimed that <tt>call_rcu()</tt> and
<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
by readers.
But how can that be correct, given that the invocation of the callback
and the freeing of the memory (respectively) must still wait for
a grace period to elapse?
<br><a href="#qq13answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Earlier it was claimed that <tt>call_rcu()</tt> and
<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
by readers.
But how can that be correct, given that the invocation of the callback
and the freeing of the memory (respectively) must still wait for
a grace period to elapse?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
We could define things this way, but keep in mind that this sort of
definition would say that updates in garbage-collected languages
cannot complete until the next time the garbage collector runs,
which does not seem at all reasonable.
The key point is that in most cases, an updater using either
<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
next update as soon as it has invoked <tt>call_rcu()</tt> or
<tt>kfree_rcu()</tt>, without having to wait for a subsequent
grace period.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
But what if the updater must wait for the completion of code to be
......@@ -1838,11 +2177,26 @@ kthreads to be spawned.
Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler
initialization can result in deadlock.
<p><a name="Quick Quiz 14"><b>Quick Quiz 14</b>:</a>
So what happens with <tt>synchronize_rcu()</tt> during
scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
kernels?
<br><a href="#qq14answer">Answer</a>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
So what happens with <tt>synchronize_rcu()</tt> during
scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
kernels?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt>
maps directly to <tt>synchronize_sched()</tt>.
Therefore, <tt>synchronize_rcu()</tt> works normally throughout
boot in <tt>CONFIG_PREEMPT=n</tt> kernels.
However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels,
so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
during scheduler initialization.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
I learned of these boot-time requirements as a result of a series of
......@@ -2170,6 +2524,14 @@ up to and including systems with 4096 CPUs.
This real-time requirement motivated the grace-period kthread, which
also simplified handling of a number of race conditions.
<p>
RCU must avoid degrading real-time response for CPU-bound threads, whether
executing in usermode (which is one use case for
<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel.
That said, CPU-bound loops in the kernel must execute
<tt>cond_resched_rcu_qs()</tt> at least once per few tens of milliseconds
in order to avoid receiving an IPI from RCU.
<p>
Finally, RCU's status as a synchronization primitive means that
any RCU failure can result in arbitrary memory corruption that can be
......@@ -2223,6 +2585,8 @@ described in a separate section.
<li> <a href="#Sched Flavor">Sched Flavor</a>
<li> <a href="#Sleepable RCU">Sleepable RCU</a>
<li> <a href="#Tasks RCU">Tasks RCU</a>
<li> <a href="#Waiting for Multiple Grace Periods">
Waiting for Multiple Grace Periods</a>
</ol>
<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
......@@ -2472,6 +2836,94 @@ The tasks-RCU API is quite compact, consisting only of
<tt>synchronize_rcu_tasks()</tt>, and
<tt>rcu_barrier_tasks()</tt>.
<h3><a name="Waiting for Multiple Grace Periods">
Waiting for Multiple Grace Periods</a></h3>
<p>
Perhaps you have an RCU protected data structure that is accessed from
RCU read-side critical sections, from softirq handlers, and from
hardware interrupt handlers.
That is three flavors of RCU, the normal flavor, the bottom-half flavor,
and the sched flavor.
How to wait for a compound grace period?
<p>
The best approach is usually to &ldquo;just say no!&rdquo; and
insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
around each RCU read-side critical section, regardless of what
environment it happens to be in.
But suppose that some of the RCU read-side critical sections are
on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt>
is not a viable option, so that <tt>rcu_read_lock()</tt> and
<tt>rcu_read_unlock()</tt> are not free.
What then?
<p>
You <i>could</i> wait on all three grace periods in succession, as follows:
<blockquote>
<pre>
1 synchronize_rcu();
2 synchronize_rcu_bh();
3 synchronize_sched();
</pre>
</blockquote>
<p>
This works, but triples the update-side latency penalty.
In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt>
may be used to wait on all three flavors of grace period concurrently:
<blockquote>
<pre>
1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
</pre>
</blockquote>
<p>
But what if it is necessary to also wait on SRCU?
This can be done as follows:
<blockquote>
<pre>
1 static void call_my_srcu(struct rcu_head *head,
2 void (*func)(struct rcu_head *head))
3 {
4 call_srcu(&amp;my_srcu, head, func);
5 }
6
7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
</pre>
</blockquote>
<p>
If you needed to wait on multiple different flavors of SRCU
(but why???), you would need to create a wrapper function resembling
<tt>call_my_srcu()</tt> for each SRCU flavor.
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But what if I need to wait for multiple RCU flavors, but I also need
the grace periods to be expedited?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
If you are using expedited grace periods, there should be less penalty
for waiting on them in succession.
But if that is nevertheless a problem, you can use workqueues
or multiple kthreads to wait on the various expedited grace
periods concurrently.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
Again, it is usually better to adjust the RCU read-side critical sections
to use a single flavor of RCU, but when this is not feasible, you can use
<tt>synchronize_rcu_mult()</tt>.
<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
<p>
......@@ -2569,329 +3021,4 @@ and is provided
under the terms of the Creative Commons Attribution-Share Alike 3.0
United States license.
<h3><a name="Answers to Quick Quizzes">
Answers to Quick Quizzes</a></h3>
<a name="qq1answer"></a>
<p><b>Quick Quiz 1</b>:
Wait a minute!
You said that updaters can make useful forward progress concurrently
with readers, but pre-existing readers will block
<tt>synchronize_rcu()</tt>!!!
Just who are you trying to fool???
</p><p><b>Answer</b>:
First, if updaters do not wish to be blocked by readers, they can use
<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
be discussed later.
Second, even when using <tt>synchronize_rcu()</tt>, the other
update-side code does run concurrently with readers, whether pre-existing
or not.
</p><p><a href="#Quick%20Quiz%201"><b>Back to Quick Quiz 1</b>.</a>
<a name="qq2answer"></a>
<p><b>Quick Quiz 2</b>:
Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
</p><p><b>Answer</b>:
Without that extra grace period, memory reordering could result in
<tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
concurrently with the last bits of <tt>recovery()</tt>.
</p><p><a href="#Quick%20Quiz%202"><b>Back to Quick Quiz 2</b>.</a>
<a name="qq3answer"></a>
<p><b>Quick Quiz 3</b>:
But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
from being reordered.
Can't that also cause problems?
</p><p><b>Answer</b>:
No, it cannot.
The readers cannot see either of these two fields until
the assignment to <tt>gp</tt>, by which time both fields are
fully initialized.
So reordering the assignments
to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt> cannot possibly
cause any problems.
</p><p><a href="#Quick%20Quiz%203"><b>Back to Quick Quiz 3</b>.</a>
<a name="qq4answer"></a>
<p><b>Quick Quiz 4</b>:
Without the <tt>rcu_dereference()</tt> or the
<tt>rcu_access_pointer()</tt>, what destructive optimizations
might the compiler make use of?
</p><p><b>Answer</b>:
Let's start with what happens to <tt>do_something_gp()</tt>
if it fails to use <tt>rcu_dereference()</tt>.
It could reuse a value formerly fetched from this same pointer.
It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
mash-up of two distince pointer values.
It might even use value-speculation optimizations, where it makes a wrong
guess, but by the time it gets around to checking the value, an update
has changed the pointer to match the wrong guess.
Too bad about any dereferences that returned pre-initialization garbage
in the meantime!
<p>
For <tt>remove_gp_synchronous()</tt>, as long as all modifications
to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
the above optimizations are harmless.
However,
with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>,
<tt>sparse</tt> will complain if you
define <tt>gp</tt> with <tt>__rcu</tt> and then
access it without using
either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
</p><p><a href="#Quick%20Quiz%204"><b>Back to Quick Quiz 4</b>.</a>
<a name="qq5answer"></a>
<p><b>Quick Quiz 5</b>:
Given that multiple CPUs can start RCU read-side critical sections
at any time without any ordering whatsoever, how can RCU possibly tell whether
or not a given RCU read-side critical section starts before a
given instance of <tt>synchronize_rcu()</tt>?
</p><p><b>Answer</b>:
If RCU cannot tell whether or not a given
RCU read-side critical section starts before a
given instance of <tt>synchronize_rcu()</tt>,
then it must assume that the RCU read-side critical section
started first.
In other words, a given instance of <tt>synchronize_rcu()</tt>
can avoid waiting on a given RCU read-side critical section only
if it can prove that <tt>synchronize_rcu()</tt> started first.
</p><p><a href="#Quick%20Quiz%205"><b>Back to Quick Quiz 5</b>.</a>
<a name="qq6answer"></a>
<p><b>Quick Quiz 6</b>:
The first and second guarantees require unbelievably strict ordering!
Are all these memory barriers <i> really</i> required?
</p><p><b>Answer</b>:
Yes, they really are required.
To see why the first guarantee is required, consider the following
sequence of events:
<ol>
<li> CPU 1: <tt>rcu_read_lock()</tt>
<li> CPU 1: <tt>q = rcu_dereference(gp);
/* Very likely to return p. */</tt>
<li> CPU 0: <tt>list_del_rcu(p);</tt>
<li> CPU 0: <tt>synchronize_rcu()</tt> starts.
<li> CPU 1: <tt>do_something_with(q-&gt;a);
/* No smp_mb(), so might happen after kfree(). */</tt>
<li> CPU 1: <tt>rcu_read_unlock()</tt>
<li> CPU 0: <tt>synchronize_rcu()</tt> returns.
<li> CPU 0: <tt>kfree(p);</tt>
</ol>
<p>
Therefore, there absolutely must be a full memory barrier between the
end of the RCU read-side critical section and the end of the
grace period.
<p>
The sequence of events demonstrating the necessity of the second rule
is roughly similar:
<ol>
<li> CPU 0: <tt>list_del_rcu(p);</tt>
<li> CPU 0: <tt>synchronize_rcu()</tt> starts.
<li> CPU 1: <tt>rcu_read_lock()</tt>
<li> CPU 1: <tt>q = rcu_dereference(gp);
/* Might return p if no memory barrier. */</tt>
<li> CPU 0: <tt>synchronize_rcu()</tt> returns.
<li> CPU 0: <tt>kfree(p);</tt>
<li> CPU 1: <tt>do_something_with(q-&gt;a); /* Boom!!! */</tt>
<li> CPU 1: <tt>rcu_read_unlock()</tt>
</ol>
<p>
And similarly, without a memory barrier between the beginning of the
grace period and the beginning of the RCU read-side critical section,
CPU&nbsp;1 might end up accessing the freelist.
<p>
The &ldquo;as if&rdquo; rule of course applies, so that any implementation
that acts as if the appropriate memory barriers were in place is a
correct implementation.
That said, it is much easier to fool yourself into believing that you have
adhered to the as-if rule than it is to actually adhere to it!
</p><p><a href="#Quick%20Quiz%206"><b>Back to Quick Quiz 6</b>.</a>
<a name="qq7answer"></a>
<p><b>Quick Quiz 7</b>:
But how does the upgrade-to-write operation exclude other readers?
</p><p><b>Answer</b>:
It doesn't, just like normal RCU updates, which also do not exclude
RCU readers.
</p><p><a href="#Quick%20Quiz%207"><b>Back to Quick Quiz 7</b>.</a>
<a name="qq8answer"></a>
<p><b>Quick Quiz 8</b>:
Can't the compiler also reorder this code?
</p><p><b>Answer</b>:
No, the volatile casts in <tt>READ_ONCE()</tt> and
<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
this particular case.
</p><p><a href="#Quick%20Quiz%208"><b>Back to Quick Quiz 8</b>.</a>
<a name="qq9answer"></a>
<p><b>Quick Quiz 9</b>:
Suppose that synchronize_rcu() did wait until all readers had completed.
Would the updater be able to rely on this?
</p><p><b>Answer</b>:
No.
Even if <tt>synchronize_rcu()</tt> were to wait until
all readers had completed, a new reader might start immediately after
<tt>synchronize_rcu()</tt> completed.
Therefore, the code following
<tt>synchronize_rcu()</tt> cannot rely on there being no readers
in any case.
</p><p><a href="#Quick%20Quiz%209"><b>Back to Quick Quiz 9</b>.</a>
<a name="qq10answer"></a>
<p><b>Quick Quiz 10</b>:
How long a sequence of grace periods, each separated by an RCU read-side
critical section, would be required to partition the RCU read-side
critical sections at the beginning and end of the chain?
</p><p><b>Answer</b>:
In theory, an infinite number.
In practice, an unknown number that is sensitive to both implementation
details and timing considerations.
Therefore, even in practice, RCU users must abide by the theoretical rather
than the practical answer.
</p><p><a href="#Quick%20Quiz%2010"><b>Back to Quick Quiz 10</b>.</a>
<a name="qq11answer"></a>
<p><b>Quick Quiz 11</b>:
What about sleeping locks?
</p><p><b>Answer</b>:
These are forbidden within Linux-kernel RCU read-side critical sections
because it is not legal to place a quiescent state (in this case,
voluntary context switch) within an RCU read-side critical section.
However, sleeping locks may be used within userspace RCU read-side critical
sections, and also within Linux-kernel sleepable RCU
<a href="#Sleepable RCU">(SRCU)</a>
read-side critical sections.
In addition, the -rt patchset turns spinlocks into a sleeping locks so
that the corresponding critical sections can be preempted, which
also means that these sleeplockified spinlocks (but not other sleeping locks!)
may be acquire within -rt-Linux-kernel RCU read-side critical sections.
<p>
Note that it <i>is</i> legal for a normal RCU read-side critical section
to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>),
but only as long as it does not loop indefinitely attempting to
conditionally acquire that sleeping locks.
The key point is that things like <tt>mutex_trylock()</tt>
either return with the mutex held, or return an error indication if
the mutex was not immediately available.
Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping.
</p><p><a href="#Quick%20Quiz%2011"><b>Back to Quick Quiz 11</b>.</a>
<a name="qq12answer"></a>
<p><b>Quick Quiz 12</b>:
Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
structure, which would interact badly with concurrent insertions.
Doesn't this mean that <tt>rcu_dereference()</tt> is required?
</p><p><b>Answer</b>:
Presumably the <tt>-&gt;gp_lock</tt> acquired on line&nbsp;18 excludes
any changes, including any insertions that <tt>rcu_dereference()</tt>
would protect against.
Therefore, any insertions will be delayed until after <tt>-&gt;gp_lock</tt>
is released on line&nbsp;25, which in turn means that
<tt>rcu_access_pointer()</tt> suffices.
</p><p><a href="#Quick%20Quiz%2012"><b>Back to Quick Quiz 12</b>.</a>
<a name="qq13answer"></a>
<p><b>Quick Quiz 13</b>:
Earlier it was claimed that <tt>call_rcu()</tt> and
<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
by readers.
But how can that be correct, given that the invocation of the callback
and the freeing of the memory (respectively) must still wait for
a grace period to elapse?
</p><p><b>Answer</b>:
We could define things this way, but keep in mind that this sort of
definition would say that updates in garbage-collected languages
cannot complete until the next time the garbage collector runs,
which does not seem at all reasonable.
The key point is that in most cases, an updater using either
<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
next update as soon as it has invoked <tt>call_rcu()</tt> or
<tt>kfree_rcu()</tt>, without having to wait for a subsequent
grace period.
</p><p><a href="#Quick%20Quiz%2013"><b>Back to Quick Quiz 13</b>.</a>
<a name="qq14answer"></a>
<p><b>Quick Quiz 14</b>:
So what happens with <tt>synchronize_rcu()</tt> during
scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
kernels?
</p><p><b>Answer</b>:
In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt>
maps directly to <tt>synchronize_sched()</tt>.
Therefore, <tt>synchronize_rcu()</tt> works normally throughout
boot in <tt>CONFIG_PREEMPT=n</tt> kernels.
However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels,
so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
during scheduler initialization.
</p><p><a href="#Quick%20Quiz%2014"><b>Back to Quick Quiz 14</b>.</a>
</body></html>
Documentation/RCU/Design/Requirements/Requirements.htmlx deleted 100644 → 0
View file @ 0052af44
This source diff could not be displayed because it is too large. You can view the blob instead.
Documentation/RCU/Design/htmlqqz.sh deleted 100755 → 0
View file @ 0052af44
#!/bin/sh
#
# Usage: sh htmlqqz.sh file
#
# Extracts and converts quick quizzes in a proto-HTML document file.htmlx.
# Commands, all of which must be on a line by themselves:
#
# "<p>@@QQ@@": Start of a quick quiz.
# "<p>@@QQA@@": Start of a quick-quiz answer.
# "<p>@@QQE@@": End of a quick-quiz answer, and thus of the quick quiz.
# "<p>@@QQAL@@": Place to put quick-quiz answer list.
#
# Places the result in file.html.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, you can access it online at
# http://www.gnu.org/licenses/gpl-2.0.html.
#
# Copyright (c) 2013 Paul E. McKenney, IBM Corporation.
fn=$1
if test ! -r $fn.htmlx
then
echo "Error: $fn.htmlx unreadable."
exit 1
fi
echo "<!-- DO NOT HAND EDIT. -->" > $fn.html
echo "<!-- Instead, edit $fn.htmlx and run 'sh htmlqqz.sh $fn' -->" >> $fn.html
awk < $fn.htmlx >> $fn.html '
state == "" && $1 != "<p>@@QQ@@" && $1 != "<p>@@QQAL@@" {
print $0;
if ($0 ~ /^<p>@@QQ/)
print "Bad Quick Quiz command: " NR " (expected <p>@@QQ@@ or <p>@@QQAL@@)." > "/dev/stderr"
next;
}
state == "" && $1 == "<p>@@QQ@@" {
qqn++;
qqlineno = NR;
haveqq = 1;
state = "qq";
print "<p><a name=\"Quick Quiz " qqn "\"><b>Quick Quiz " qqn "</b>:</a>"
next;
}
state == "qq" && $1 != "<p>@@QQA@@" {
qq[qqn] = qq[qqn] $0 "\n";
print $0
if ($0 ~ /^<p>@@QQ/)
print "Bad Quick Quiz command: " NR ". (expected <p>@@QQA@@)" > "/dev/stderr"
next;
}
state == "qq" && $1 == "<p>@@QQA@@" {
state = "qqa";
print "<br><a href=\"#qq" qqn "answer\">Answer</a>"
next;
}
state == "qqa" && $1 != "<p>@@QQE@@" {
qqa[qqn] = qqa[qqn] $0 "\n";
if ($0 ~ /^<p>@@QQ/)
print "Bad Quick Quiz command: " NR " (expected <p>@@QQE@@)." > "/dev/stderr"
next;
}
state == "qqa" && $1 == "<p>@@QQE@@" {
state = "";
next;
}
state == "" && $1 == "<p>@@QQAL@@" {
haveqq = "";
print "<h3><a name=\"Answers to Quick Quizzes\">"
print "Answers to Quick Quizzes</a></h3>"
print "";
for (i = 1; i <= qqn; i++) {
print "<a name=\"qq" i "answer\"></a>"
print "<p><b>Quick Quiz " i "</b>:"
print qq[i];
print "";
print "</p><p><b>Answer</b>:"
print qqa[i];
print "";
print "</p><p><a href=\"#Quick%20Quiz%20" i "\"><b>Back to Quick Quiz " i "</b>.</a>"
print "";
}
next;
}
END {
if (state != "")
print "Unterminated Quick Quiz: " qqlineno "." > "/dev/stderr"
else if (haveqq)
print "Missing \"<p>@@QQAL@@\", no Quick Quiz." > "/dev/stderr"
}'
Documentation/RCU/trace.txt
View file @ a3871bd4
......@@ -237,17 +237,17 @@ o "ktl" is the low-order 16 bits (in hexadecimal) of the count of
The output of "cat rcu/rcu_preempt/rcuexp" looks as follows:
s=21872 wd0=0 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872
s=21872 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872
These fields are as follows:
o "s" is the sequence number, with an odd number indicating that
an expedited grace period is in progress.
o "wd0", "wd1", "wd2", and "wd3" are the number of times that an
attempt to start an expedited grace period found that someone
else had completed an expedited grace period that satisfies the
attempted request. "Our work is done."
o "wd1", "wd2", and "wd3" are the number of times that an attempt
to start an expedited grace period found that someone else had
completed an expedited grace period that satisfies the attempted
request. "Our work is done."
o "n" is number of times that a concurrent CPU-hotplug operation
forced a fallback to a normal grace period.
......
Documentation/RCU/whatisRCU.txt
View file @ a3871bd4
......@@ -681,22 +681,30 @@ Although RCU can be used in many different ways, a very common use of
RCU is analogous to reader-writer locking. The following unified
diff shows how closely related RCU and reader-writer locking can be.
@@ -5,5 +5,5 @@ struct el {
int data;
/* Other data fields */
};
-rwlock_t listmutex;
+spinlock_t listmutex;
struct el head;
@@ -13,15 +14,15 @@
struct list_head *lp;
struct el *p;
- read_lock();
- read_lock(&listmutex);
- list_for_each_entry(p, head, lp) {
+ rcu_read_lock();
+ list_for_each_entry_rcu(p, head, lp) {
if (p->key == key) {
*result = p->data;
- read_unlock();
- read_unlock(&listmutex);
+ rcu_read_unlock();
return 1;
}
}
- read_unlock();
- read_unlock(&listmutex);
+ rcu_read_unlock();
return 0;
}
......@@ -732,7 +740,7 @@ Or, for those who prefer a side-by-side listing:
5 int data; 5 int data;
6 /* Other data fields */ 6 /* Other data fields */
7 }; 7 };
8 spinlock_t listmutex; 8 spinlock_t listmutex;
8 rwlock_t listmutex; 8 spinlock_t listmutex;
9 struct el head; 9 struct el head;
1 int search(long key, int *result) 1 int search(long key, int *result)
......@@ -740,15 +748,15 @@ Or, for those who prefer a side-by-side listing:
3 struct list_head *lp; 3 struct list_head *lp;
4 struct el *p; 4 struct el *p;
5 5
6 read_lock(); 6 rcu_read_lock();
6 read_lock(&listmutex); 6 rcu_read_lock();
7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) {
8 if (p->key == key) { 8 if (p->key == key) {
9 *result = p->data; 9 *result = p->data;
10 read_unlock(); 10 rcu_read_unlock();
10 read_unlock(&listmutex); 10 rcu_read_unlock();
11 return 1; 11 return 1;
12 } 12 }
13 } 13 }
14 read_unlock(); 14 rcu_read_unlock();
14 read_unlock(&listmutex); 14 rcu_read_unlock();
15 return 0; 15 return 0;
16 } 16 }
......
Documentation/kernel-parameters.txt
View file @ a3871bd4
......@@ -3284,6 +3284,44 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
Lazy RCU callbacks are those which RCU can
prove do nothing more than free memory.
rcuperf.gp_exp= [KNL]
Measure performance of expedited synchronous
grace-period primitives.
rcuperf.holdoff= [KNL]
Set test-start holdoff period. The purpose of
this parameter is to delay the start of the
test until boot completes in order to avoid
interference.
rcuperf.nreaders= [KNL]
Set number of RCU readers. The value -1 selects
N, where N is the number of CPUs. A value
"n" less than -1 selects N-n+1, where N is again
the number of CPUs. For example, -2 selects N
(the number of CPUs), -3 selects N+1, and so on.
A value of "n" less than or equal to -N selects
a single reader.
rcuperf.nwriters= [KNL]
Set number of RCU writers. The values operate
the same as for rcuperf.nreaders.
N, where N is the number of CPUs
rcuperf.perf_runnable= [BOOT]
Start rcuperf running at boot time.
rcuperf.shutdown= [KNL]
Shut the system down after performance tests
complete. This is useful for hands-off automated
testing.
rcuperf.perf_type= [KNL]
Specify the RCU implementation to test.
rcuperf.verbose= [KNL]
Enable additional printk() statements.
rcutorture.cbflood_inter_holdoff= [KNL]
Set holdoff time (jiffies) between successive
callback-flood tests.
......
include/linux/rcupdate.h
View file @ a3871bd4
......@@ -508,14 +508,7 @@ int rcu_read_lock_bh_held(void);
* CONFIG_DEBUG_LOCK_ALLOC, this assumes we are in an RCU-sched read-side
* critical section unless it can prove otherwise.
*/
#ifdef CONFIG_PREEMPT_COUNT
int rcu_read_lock_sched_held(void);
#else /* #ifdef CONFIG_PREEMPT_COUNT */
static inline int rcu_read_lock_sched_held(void)
{
return 1;
}
#endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
......@@ -532,18 +525,10 @@ static inline int rcu_read_lock_bh_held(void)
return 1;
}
#ifdef CONFIG_PREEMPT_COUNT
static inline int rcu_read_lock_sched_held(void)
{
return preempt_count() != 0 || irqs_disabled();
}
#else /* #ifdef CONFIG_PREEMPT_COUNT */
static inline int rcu_read_lock_sched_held(void)
{
return 1;
return !preemptible();
}
#endif /* #else #ifdef CONFIG_PREEMPT_COUNT */
#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
#ifdef CONFIG_PROVE_RCU
......@@ -1144,4 +1129,17 @@ static inline void rcu_sysidle_force_exit(void)
#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
/*
* Dump the ftrace buffer, but only one time per callsite per boot.
*/
#define rcu_ftrace_dump(oops_dump_mode) \
do { \
static atomic_t ___rfd_beenhere = ATOMIC_INIT(0); \
\
if (!atomic_read(&___rfd_beenhere) && \
!atomic_xchg(&___rfd_beenhere, 1)) \
ftrace_dump(oops_dump_mode); \
} while (0)
#endif /* __LINUX_RCUPDATE_H */
include/linux/rcutiny.h
View file @ a3871bd4
......@@ -149,6 +149,22 @@ static inline unsigned long rcu_batches_completed_sched(void)
return 0;
}
/*
* Return the number of expedited grace periods completed.
*/
static inline unsigned long rcu_exp_batches_completed(void)
{
return 0;
}
/*
* Return the number of expedited sched grace periods completed.
*/
static inline unsigned long rcu_exp_batches_completed_sched(void)
{
return 0;
}
static inline void rcu_force_quiescent_state(void)
{
}
......
include/linux/rcutree.h
View file @ a3871bd4
......@@ -87,6 +87,8 @@ unsigned long rcu_batches_started_sched(void);
unsigned long rcu_batches_completed(void);
unsigned long rcu_batches_completed_bh(void);
unsigned long rcu_batches_completed_sched(void);
unsigned long rcu_exp_batches_completed(void);
unsigned long rcu_exp_batches_completed_sched(void);
void show_rcu_gp_kthreads(void);
void rcu_force_quiescent_state(void);
......
include/trace/events/rcu.h
View file @ a3871bd4
......@@ -171,6 +171,77 @@ TRACE_EVENT(rcu_grace_period_init,
__entry->grplo, __entry->grphi, __entry->qsmask)
);
/*
* Tracepoint for expedited grace-period events. Takes a string identifying
* the RCU flavor, the expedited grace-period sequence number, and a string
* identifying the grace-period-related event as follows:
*
* "snap": Captured snapshot of expedited grace period sequence number.
* "start": Started a real expedited grace period.
* "end": Ended a real expedited grace period.
* "endwake": Woke piggybackers up.
* "done": Someone else did the expedited grace period for us.
*/
TRACE_EVENT(rcu_exp_grace_period,
TP_PROTO(const char *rcuname, unsigned long gpseq, const char *gpevent),
TP_ARGS(rcuname, gpseq, gpevent),
TP_STRUCT__entry(
__field(const char *, rcuname)
__field(unsigned long, gpseq)
__field(const char *, gpevent)
),
TP_fast_assign(
__entry->rcuname = rcuname;
__entry->gpseq = gpseq;
__entry->gpevent = gpevent;
),
TP_printk("%s %lu %s",
__entry->rcuname, __entry->gpseq, __entry->gpevent)
);
/*
* Tracepoint for expedited grace-period funnel-locking events. Takes a
* string identifying the RCU flavor, an integer identifying the rcu_node
* combining-tree level, another pair of integers identifying the lowest-
* and highest-numbered CPU associated with the current rcu_node structure,
* and a string. identifying the grace-period-related event as follows:
*
* "nxtlvl": Advance to next level of rcu_node funnel
* "wait": Wait for someone else to do expedited GP
*/
TRACE_EVENT(rcu_exp_funnel_lock,
TP_PROTO(const char *rcuname, u8 level, int grplo, int grphi,
const char *gpevent),
TP_ARGS(rcuname, level, grplo, grphi, gpevent),
TP_STRUCT__entry(
__field(const char *, rcuname)
__field(u8, level)
__field(int, grplo)
__field(int, grphi)
__field(const char *, gpevent)
),
TP_fast_assign(
__entry->rcuname = rcuname;
__entry->level = level;
__entry->grplo = grplo;
__entry->grphi = grphi;
__entry->gpevent = gpevent;
),
TP_printk("%s %d %d %d %s",
__entry->rcuname, __entry->level, __entry->grplo,
__entry->grphi, __entry->gpevent)
);
/*
* Tracepoint for RCU no-CBs CPU callback handoffs. This event is intended
* to assist debugging of these handoffs.
......@@ -704,11 +775,15 @@ TRACE_EVENT(rcu_barrier,
#else /* #ifdef CONFIG_RCU_TRACE */
#define trace_rcu_grace_period(rcuname, gpnum, gpevent) do { } while (0)
#define trace_rcu_grace_period_init(rcuname, gpnum, level, grplo, grphi, \
qsmask) do { } while (0)
#define trace_rcu_future_grace_period(rcuname, gpnum, completed, c, \
level, grplo, grphi, event) \
do { } while (0)
#define trace_rcu_grace_period_init(rcuname, gpnum, level, grplo, grphi, \
qsmask) do { } while (0)
#define trace_rcu_exp_grace_period(rcuname, gqseq, gpevent) \
do { } while (0)
#define trace_rcu_exp_funnel_lock(rcuname, level, grplo, grphi, gpevent) \
do { } while (0)
#define trace_rcu_nocb_wake(rcuname, cpu, reason) do { } while (0)
#define trace_rcu_preempt_task(rcuname, pid, gpnum) do { } while (0)
#define trace_rcu_unlock_preempted_task(rcuname, gpnum, pid) do { } while (0)
......
kernel/rcu/Makefile
View file @ a3871bd4
......@@ -5,6 +5,7 @@ KCOV_INSTRUMENT := n
obj-y += update.o sync.o
obj-$(CONFIG_SRCU) += srcu.o
obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
obj-$(CONFIG_RCU_PERF_TEST) += rcuperf.o
obj-$(CONFIG_TREE_RCU) += tree.o
obj-$(CONFIG_PREEMPT_RCU) += tree.o
obj-$(CONFIG_TREE_RCU_TRACE) += tree_trace.o
......
kernel/rcu/rcuperf.c 0 → 100644
View file @ a3871bd4
/*
* Read-Copy Update module-based performance-test facility
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, you can access it online at
* http://www.gnu.org/licenses/gpl-2.0.html.
*
* Copyright (C) IBM Corporation, 2015
*
* Authors: Paul E. McKenney <paulmck@us.ibm.com>
*/
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kthread.h>
#include <linux/err.h>
#include <linux/spinlock.h>
#include <linux/smp.h>
#include <linux/rcupdate.h>
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <linux/atomic.h>
#include <linux/bitops.h>
#include <linux/completion.h>
#include <linux/moduleparam.h>
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/reboot.h>
#include <linux/freezer.h>
#include <linux/cpu.h>
#include <linux/delay.h>
#include <linux/stat.h>
#include <linux/srcu.h>
#include <linux/slab.h>
#include <asm/byteorder.h>
#include <linux/torture.h>
#include <linux/vmalloc.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Paul E. McKenney <paulmck@linux.vnet.ibm.com>");
#define PERF_FLAG "-perf:"
#define PERFOUT_STRING(s) \
pr_alert("%s" PERF_FLAG s "\n", perf_type)
#define VERBOSE_PERFOUT_STRING(s) \
do { if (verbose) pr_alert("%s" PERF_FLAG " %s\n", perf_type, s); } while (0)
#define VERBOSE_PERFOUT_ERRSTRING(s) \
do { if (verbose) pr_alert("%s" PERF_FLAG "!!! %s\n", perf_type, s); } while (0)
torture_param(bool, gp_exp, true, "Use expedited GP wait primitives");
torture_param(int, holdoff, 10, "Holdoff time before test start (s)");
torture_param(int, nreaders, -1, "Number of RCU reader threads");
torture_param(int, nwriters, -1, "Number of RCU updater threads");
torture_param(bool, shutdown, false, "Shutdown at end of performance tests.");
torture_param(bool, verbose, true, "Enable verbose debugging printk()s");
static char *perf_type = "rcu";
module_param(perf_type, charp, 0444);
MODULE_PARM_DESC(perf_type, "Type of RCU to performance-test (rcu, rcu_bh, ...)");
static int nrealreaders;
static int nrealwriters;
static struct task_struct **writer_tasks;
static struct task_struct **reader_tasks;
static struct task_struct *shutdown_task;
static u64 **writer_durations;
static int *writer_n_durations;
static atomic_t n_rcu_perf_reader_started;
static atomic_t n_rcu_perf_writer_started;
static atomic_t n_rcu_perf_writer_finished;
static wait_queue_head_t shutdown_wq;
static u64 t_rcu_perf_writer_started;
static u64 t_rcu_perf_writer_finished;
static unsigned long b_rcu_perf_writer_started;
static unsigned long b_rcu_perf_writer_finished;
static int rcu_perf_writer_state;
#define RTWS_INIT 0
#define RTWS_EXP_SYNC 1
#define RTWS_SYNC 2
#define RTWS_IDLE 2
#define RTWS_STOPPING 3
#define MAX_MEAS 10000
#define MIN_MEAS 100
#if defined(MODULE) || defined(CONFIG_RCU_PERF_TEST_RUNNABLE)
#define RCUPERF_RUNNABLE_INIT 1
#else
#define RCUPERF_RUNNABLE_INIT 0
#endif
static int perf_runnable = RCUPERF_RUNNABLE_INIT;
module_param(perf_runnable, int, 0444);
MODULE_PARM_DESC(perf_runnable, "Start rcuperf at boot");
/*
* Operations vector for selecting different types of tests.
*/
struct rcu_perf_ops {
int ptype;
void (*init)(void);
void (*cleanup)(void);
int (*readlock)(void);
void (*readunlock)(int idx);
unsigned long (*started)(void);
unsigned long (*completed)(void);
unsigned long (*exp_completed)(void);
void (*sync)(void);
void (*exp_sync)(void);
const char *name;
};
static struct rcu_perf_ops *cur_ops;
/*
* Definitions for rcu perf testing.
*/
static int rcu_perf_read_lock(void) __acquires(RCU)
{
rcu_read_lock();
return 0;
}
static void rcu_perf_read_unlock(int idx) __releases(RCU)
{
rcu_read_unlock();
}
static unsigned long __maybe_unused rcu_no_completed(void)
{
return 0;
}
static void rcu_sync_perf_init(void)
{
}
static struct rcu_perf_ops rcu_ops = {
.ptype = RCU_FLAVOR,
.init = rcu_sync_perf_init,
.readlock = rcu_perf_read_lock,
.readunlock = rcu_perf_read_unlock,
.started = rcu_batches_started,
.completed = rcu_batches_completed,
.exp_completed = rcu_exp_batches_completed,
.sync = synchronize_rcu,
.exp_sync = synchronize_rcu_expedited,
.name = "rcu"
};
/*
* Definitions for rcu_bh perf testing.
*/
static int rcu_bh_perf_read_lock(void) __acquires(RCU_BH)
{
rcu_read_lock_bh();
return 0;
}
static void rcu_bh_perf_read_unlock(int idx) __releases(RCU_BH)
{
rcu_read_unlock_bh();
}
static struct rcu_perf_ops rcu_bh_ops = {
.ptype = RCU_BH_FLAVOR,
.init = rcu_sync_perf_init,
.readlock = rcu_bh_perf_read_lock,
.readunlock = rcu_bh_perf_read_unlock,
.started = rcu_batches_started_bh,
.completed = rcu_batches_completed_bh,
.exp_completed = rcu_exp_batches_completed_sched,
.sync = synchronize_rcu_bh,
.exp_sync = synchronize_rcu_bh_expedited,
.name = "rcu_bh"
};
/*
* Definitions for srcu perf testing.
*/
DEFINE_STATIC_SRCU(srcu_ctl_perf);
static struct srcu_struct *srcu_ctlp = &srcu_ctl_perf;
static int srcu_perf_read_lock(void) __acquires(srcu_ctlp)
{
return srcu_read_lock(srcu_ctlp);
}
static void srcu_perf_read_unlock(int idx) __releases(srcu_ctlp)
{
srcu_read_unlock(srcu_ctlp, idx);
}
static unsigned long srcu_perf_completed(void)
{
return srcu_batches_completed(srcu_ctlp);
}
static void srcu_perf_synchronize(void)
{
synchronize_srcu(srcu_ctlp);
}
static void srcu_perf_synchronize_expedited(void)
{
synchronize_srcu_expedited(srcu_ctlp);
}
static struct rcu_perf_ops srcu_ops = {
.ptype = SRCU_FLAVOR,
.init = rcu_sync_perf_init,
.readlock = srcu_perf_read_lock,
.readunlock = srcu_perf_read_unlock,
.started = NULL,
.completed = srcu_perf_completed,
.exp_completed = srcu_perf_completed,
.sync = srcu_perf_synchronize,
.exp_sync = srcu_perf_synchronize_expedited,
.name = "srcu"
};
/*
* Definitions for sched perf testing.
*/
static int sched_perf_read_lock(void)
{
preempt_disable();
return 0;
}
static void sched_perf_read_unlock(int idx)
{
preempt_enable();
}
static struct rcu_perf_ops sched_ops = {
.ptype = RCU_SCHED_FLAVOR,
.init = rcu_sync_perf_init,
.readlock = sched_perf_read_lock,
.readunlock = sched_perf_read_unlock,
.started = rcu_batches_started_sched,
.completed = rcu_batches_completed_sched,
.exp_completed = rcu_exp_batches_completed_sched,
.sync = synchronize_sched,
.exp_sync = synchronize_sched_expedited,
.name = "sched"
};
#ifdef CONFIG_TASKS_RCU
/*
* Definitions for RCU-tasks perf testing.
*/
static int tasks_perf_read_lock(void)
{
return 0;
}
static void tasks_perf_read_unlock(int idx)
{
}
static struct rcu_perf_ops tasks_ops = {
.ptype = RCU_TASKS_FLAVOR,
.init = rcu_sync_perf_init,
.readlock = tasks_perf_read_lock,
.readunlock = tasks_perf_read_unlock,
.started = rcu_no_completed,
.completed = rcu_no_completed,
.sync = synchronize_rcu_tasks,
.exp_sync = synchronize_rcu_tasks,
.name = "tasks"
};
#define RCUPERF_TASKS_OPS &tasks_ops,
static bool __maybe_unused torturing_tasks(void)
{
return cur_ops == &tasks_ops;
}
#else /* #ifdef CONFIG_TASKS_RCU */
#define RCUPERF_TASKS_OPS
static bool __maybe_unused torturing_tasks(void)
{
return false;
}
#endif /* #else #ifdef CONFIG_TASKS_RCU */
/*
* If performance tests complete, wait for shutdown to commence.
*/
static void rcu_perf_wait_shutdown(void)
{
cond_resched_rcu_qs();
if (atomic_read(&n_rcu_perf_writer_finished) < nrealwriters)
return;
while (!torture_must_stop())
schedule_timeout_uninterruptible(1);
}
/*
* RCU perf reader kthread. Repeatedly does empty RCU read-side
* critical section, minimizing update-side interference.
*/
static int
rcu_perf_reader(void *arg)
{
unsigned long flags;
int idx;
long me = (long)arg;
VERBOSE_PERFOUT_STRING("rcu_perf_reader task started");
set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
set_user_nice(current, MAX_NICE);
atomic_inc(&n_rcu_perf_reader_started);
do {
local_irq_save(flags);
idx = cur_ops->readlock();
cur_ops->readunlock(idx);
local_irq_restore(flags);
rcu_perf_wait_shutdown();
} while (!torture_must_stop());
torture_kthread_stopping("rcu_perf_reader");
return 0;
}
/*
* RCU perf writer kthread. Repeatedly does a grace period.
*/
static int
rcu_perf_writer(void *arg)
{
int i = 0;
int i_max;
long me = (long)arg;
struct sched_param sp;
bool started = false, done = false, alldone = false;
u64 t;
u64 *wdp;
u64 *wdpp = writer_durations[me];
VERBOSE_PERFOUT_STRING("rcu_perf_writer task started");
WARN_ON(rcu_gp_is_expedited() && !rcu_gp_is_normal() && !gp_exp);
WARN_ON(rcu_gp_is_normal() && gp_exp);
WARN_ON(!wdpp);
set_cpus_allowed_ptr(current, cpumask_of(me % nr_cpu_ids));
sp.sched_priority = 1;
sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
if (holdoff)
schedule_timeout_uninterruptible(holdoff * HZ);
t = ktime_get_mono_fast_ns();
if (atomic_inc_return(&n_rcu_perf_writer_started) >= nrealwriters) {
t_rcu_perf_writer_started = t;
if (gp_exp) {
b_rcu_perf_writer_started =
cur_ops->exp_completed() / 2;
} else {
b_rcu_perf_writer_started =
cur_ops->completed();
}
}
do {
wdp = &wdpp[i];
*wdp = ktime_get_mono_fast_ns();
if (gp_exp) {
rcu_perf_writer_state = RTWS_EXP_SYNC;
cur_ops->exp_sync();
} else {
rcu_perf_writer_state = RTWS_SYNC;
cur_ops->sync();
}
rcu_perf_writer_state = RTWS_IDLE;
t = ktime_get_mono_fast_ns();
*wdp = t - *wdp;
i_max = i;
if (!started &&
atomic_read(&n_rcu_perf_writer_started) >= nrealwriters)
started = true;
if (!done && i >= MIN_MEAS) {
done = true;
sp.sched_priority = 0;
sched_setscheduler_nocheck(current,
SCHED_NORMAL, &sp);
pr_alert("%s" PERF_FLAG
"rcu_perf_writer %ld has %d measurements\n",
perf_type, me, MIN_MEAS);
if (atomic_inc_return(&n_rcu_perf_writer_finished) >=
nrealwriters) {
schedule_timeout_interruptible(10);
rcu_ftrace_dump(DUMP_ALL);
PERFOUT_STRING("Test complete");
t_rcu_perf_writer_finished = t;
if (gp_exp) {
b_rcu_perf_writer_finished =
cur_ops->exp_completed() / 2;
} else {
b_rcu_perf_writer_finished =
cur_ops->completed();
}
if (shutdown) {
smp_mb(); /* Assign before wake. */
wake_up(&shutdown_wq);
}
}
}
if (done && !alldone &&
atomic_read(&n_rcu_perf_writer_finished) >= nrealwriters)
alldone = true;
if (started && !alldone && i < MAX_MEAS - 1)
i++;
rcu_perf_wait_shutdown();
} while (!torture_must_stop());
rcu_perf_writer_state = RTWS_STOPPING;
writer_n_durations[me] = i_max;
torture_kthread_stopping("rcu_perf_writer");
return 0;
}
static inline void
rcu_perf_print_module_parms(struct rcu_perf_ops *cur_ops, const char *tag)
{
pr_alert("%s" PERF_FLAG
"--- %s: nreaders=%d nwriters=%d verbose=%d shutdown=%d\n",
perf_type, tag, nrealreaders, nrealwriters, verbose, shutdown);
}
static void
rcu_perf_cleanup(void)
{
int i;
int j;
int ngps = 0;
u64 *wdp;
u64 *wdpp;
if (torture_cleanup_begin())
return;
if (reader_tasks) {
for (i = 0; i < nrealreaders; i++)
torture_stop_kthread(rcu_perf_reader,
reader_tasks[i]);
kfree(reader_tasks);
}
if (writer_tasks) {
for (i = 0; i < nrealwriters; i++) {
torture_stop_kthread(rcu_perf_writer,
writer_tasks[i]);
if (!writer_n_durations)
continue;
j = writer_n_durations[i];
pr_alert("%s%s writer %d gps: %d\n",
perf_type, PERF_FLAG, i, j);
ngps += j;
}
pr_alert("%s%s start: %llu end: %llu duration: %llu gps: %d batches: %ld\n",
perf_type, PERF_FLAG,
t_rcu_perf_writer_started, t_rcu_perf_writer_finished,
t_rcu_perf_writer_finished -
t_rcu_perf_writer_started,
ngps,
b_rcu_perf_writer_finished -
b_rcu_perf_writer_started);
for (i = 0; i < nrealwriters; i++) {
if (!writer_durations)
break;
if (!writer_n_durations)
continue;
wdpp = writer_durations[i];
if (!wdpp)
continue;
for (j = 0; j <= writer_n_durations[i]; j++) {
wdp = &wdpp[j];
pr_alert("%s%s %4d writer-duration: %5d %llu\n",
perf_type, PERF_FLAG,
i, j, *wdp);
if (j % 100 == 0)
schedule_timeout_uninterruptible(1);
}
kfree(writer_durations[i]);
}
kfree(writer_tasks);
kfree(writer_durations);
kfree(writer_n_durations);
}
/* Do flavor-specific cleanup operations. */
if (cur_ops->cleanup != NULL)
cur_ops->cleanup();
torture_cleanup_end();
}
/*
* Return the number if non-negative. If -1, the number of CPUs.
* If less than -1, that much less than the number of CPUs, but
* at least one.
*/
static int compute_real(int n)
{
int nr;
if (n >= 0) {
nr = n;
} else {
nr = num_online_cpus() + 1 + n;
if (nr <= 0)
nr = 1;
}
return nr;
}
/*
* RCU perf shutdown kthread. Just waits to be awakened, then shuts
* down system.
*/
static int
rcu_perf_shutdown(void *arg)
{
do {
wait_event(shutdown_wq,
atomic_read(&n_rcu_perf_writer_finished) >=
nrealwriters);
} while (atomic_read(&n_rcu_perf_writer_finished) < nrealwriters);
smp_mb(); /* Wake before output. */
rcu_perf_cleanup();
kernel_power_off();
return -EINVAL;
}
static int __init
rcu_perf_init(void)
{
long i;
int firsterr = 0;
static struct rcu_perf_ops *perf_ops[] = {
&rcu_ops, &rcu_bh_ops, &srcu_ops, &sched_ops,
RCUPERF_TASKS_OPS
};
if (!torture_init_begin(perf_type, verbose, &perf_runnable))
return -EBUSY;
/* Process args and tell the world that the perf'er is on the job. */
for (i = 0; i < ARRAY_SIZE(perf_ops); i++) {
cur_ops = perf_ops[i];
if (strcmp(perf_type, cur_ops->name) == 0)
break;
}
if (i == ARRAY_SIZE(perf_ops)) {
pr_alert("rcu-perf: invalid perf type: \"%s\"\n",
perf_type);
pr_alert("rcu-perf types:");
for (i = 0; i < ARRAY_SIZE(perf_ops); i++)
pr_alert(" %s", perf_ops[i]->name);
pr_alert("\n");
firsterr = -EINVAL;
goto unwind;
}
if (cur_ops->init)
cur_ops->init();
nrealwriters = compute_real(nwriters);
nrealreaders = compute_real(nreaders);
atomic_set(&n_rcu_perf_reader_started, 0);
atomic_set(&n_rcu_perf_writer_started, 0);
atomic_set(&n_rcu_perf_writer_finished, 0);
rcu_perf_print_module_parms(cur_ops, "Start of test");
/* Start up the kthreads. */
if (shutdown) {
init_waitqueue_head(&shutdown_wq);
firsterr = torture_create_kthread(rcu_perf_shutdown, NULL,
shutdown_task);
if (firsterr)
goto unwind;
schedule_timeout_uninterruptible(1);
}
reader_tasks = kcalloc(nrealreaders, sizeof(reader_tasks[0]),
GFP_KERNEL);
if (reader_tasks == NULL) {
VERBOSE_PERFOUT_ERRSTRING("out of memory");
firsterr = -ENOMEM;
goto unwind;
}
for (i = 0; i < nrealreaders; i++) {
firsterr = torture_create_kthread(rcu_perf_reader, (void *)i,
reader_tasks[i]);
if (firsterr)
goto unwind;
}
while (atomic_read(&n_rcu_perf_reader_started) < nrealreaders)
schedule_timeout_uninterruptible(1);
writer_tasks = kcalloc(nrealwriters, sizeof(reader_tasks[0]),
GFP_KERNEL);
writer_durations = kcalloc(nrealwriters, sizeof(*writer_durations),
GFP_KERNEL);
writer_n_durations =
kcalloc(nrealwriters, sizeof(*writer_n_durations),
GFP_KERNEL);
if (!writer_tasks || !writer_durations || !writer_n_durations) {
VERBOSE_PERFOUT_ERRSTRING("out of memory");
firsterr = -ENOMEM;
goto unwind;
}
for (i = 0; i < nrealwriters; i++) {
writer_durations[i] =
kcalloc(MAX_MEAS, sizeof(*writer_durations[i]),
GFP_KERNEL);
if (!writer_durations[i])
goto unwind;
firsterr = torture_create_kthread(rcu_perf_writer, (void *)i,
writer_tasks[i]);
if (firsterr)
goto unwind;
}
torture_init_end();
return 0;
unwind:
torture_init_end();
rcu_perf_cleanup();
return firsterr;
}
module_init(rcu_perf_init);
module_exit(rcu_perf_cleanup);
kernel/rcu/rcutorture.c
View file @ a3871bd4
......@@ -130,8 +130,8 @@ static struct rcu_torture __rcu *rcu_torture_current;
static unsigned long rcu_torture_current_version;
static struct rcu_torture rcu_tortures[10 * RCU_TORTURE_PIPE_LEN];
static DEFINE_SPINLOCK(rcu_torture_lock);
static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_count) = { 0 };
static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_batch) = { 0 };
static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_count);
static DEFINE_PER_CPU(long [RCU_TORTURE_PIPE_LEN + 1], rcu_torture_batch);
static atomic_t rcu_torture_wcount[RCU_TORTURE_PIPE_LEN + 1];
static atomic_t n_rcu_torture_alloc;
static atomic_t n_rcu_torture_alloc_fail;
......@@ -916,7 +916,7 @@ rcu_torture_fqs(void *arg)
static int
rcu_torture_writer(void *arg)
{
bool can_expedite = !rcu_gp_is_expedited();
bool can_expedite = !rcu_gp_is_expedited() && !rcu_gp_is_normal();
int expediting = 0;
unsigned long gp_snap;
bool gp_cond1 = gp_cond, gp_exp1 = gp_exp, gp_normal1 = gp_normal;
......@@ -932,7 +932,7 @@ rcu_torture_writer(void *arg)
VERBOSE_TOROUT_STRING("rcu_torture_writer task started");
if (!can_expedite) {
pr_alert("%s" TORTURE_FLAG
" Grace periods expedited from boot/sysfs for %s,\n",
" GP expediting controlled from boot/sysfs for %s,\n",
torture_type, cur_ops->name);
pr_alert("%s" TORTURE_FLAG
" Disabled dynamic grace-period expediting.\n",
......@@ -1082,17 +1082,6 @@ rcu_torture_fakewriter(void *arg)
return 0;
}
static void rcutorture_trace_dump(void)
{
static atomic_t beenhere = ATOMIC_INIT(0);
if (atomic_read(&beenhere))
return;
if (atomic_xchg(&beenhere, 1) != 0)
return;
ftrace_dump(DUMP_ALL);
}
/*
* RCU torture reader from timer handler. Dereferences rcu_torture_current,
* incrementing the corresponding element of the pipeline array. The
......@@ -1142,7 +1131,7 @@ static void rcu_torture_timer(unsigned long unused)
if (pipe_count > 1) {
do_trace_rcu_torture_read(cur_ops->name, &p->rtort_rcu, ts,
started, completed);
rcutorture_trace_dump();
rcu_ftrace_dump(DUMP_ALL);
}
__this_cpu_inc(rcu_torture_count[pipe_count]);
completed = completed - started;
......@@ -1215,7 +1204,7 @@ rcu_torture_reader(void *arg)
if (pipe_count > 1) {
do_trace_rcu_torture_read(cur_ops->name, &p->rtort_rcu,
ts, started, completed);
rcutorture_trace_dump();
rcu_ftrace_dump(DUMP_ALL);
}
__this_cpu_inc(rcu_torture_count[pipe_count]);
completed = completed - started;
......@@ -1333,7 +1322,7 @@ rcu_torture_stats_print(void)
rcu_torture_writer_state,
gpnum, completed, flags);
show_rcu_gp_kthreads();
rcutorture_trace_dump();
rcu_ftrace_dump(DUMP_ALL);
}
rtcv_snap = rcu_torture_current_version;
}
......@@ -1489,7 +1478,9 @@ static int rcu_torture_barrier_cbs(void *arg)
* The above smp_load_acquire() ensures barrier_phase load
* is ordered before the folloiwng ->call().
*/
local_irq_disable(); /* Just to test no-irq call_rcu(). */
cur_ops->call(&rcu, rcu_torture_barrier_cbf);
local_irq_enable();
if (atomic_dec_and_test(&barrier_cbs_count))
wake_up(&barrier_wq);
} while (!torture_must_stop());
......@@ -1596,7 +1587,7 @@ static int rcutorture_cpu_notify(struct notifier_block *self,
{
long cpu = (long)hcpu;
switch (action) {
switch (action & ~CPU_TASKS_FROZEN) {
case CPU_ONLINE:
case CPU_DOWN_FAILED:
(void)rcutorture_booster_init(cpu);
......
kernel/rcu/tree.c
View file @ a3871bd4
......@@ -102,6 +102,8 @@ struct rcu_state sname##_state = { \
.barrier_mutex = __MUTEX_INITIALIZER(sname##_state.barrier_mutex), \
.name = RCU_STATE_NAME(sname), \
.abbr = sabbr, \
.exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \
.exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \
}
RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
......@@ -370,6 +372,21 @@ void rcu_all_qs(void)
rcu_momentary_dyntick_idle();
local_irq_restore(flags);
}
if (unlikely(raw_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))) {
/*
* Yes, we just checked a per-CPU variable with preemption
* enabled, so we might be migrated to some other CPU at
* this point. That is OK because in that case, the
* migration will supply the needed quiescent state.
* We might end up needlessly disabling preemption and
* invoking rcu_sched_qs() on the destination CPU, but
* the probability and cost are both quite low, so this
* should not be a problem in practice.
*/
preempt_disable();
rcu_sched_qs();
preempt_enable();
}
this_cpu_inc(rcu_qs_ctr);
barrier(); /* Avoid RCU read-side critical sections leaking up. */
}
......@@ -385,9 +402,11 @@ module_param(qlowmark, long, 0444);
static ulong jiffies_till_first_fqs = ULONG_MAX;
static ulong jiffies_till_next_fqs = ULONG_MAX;
static bool rcu_kick_kthreads;
module_param(jiffies_till_first_fqs, ulong, 0644);
module_param(jiffies_till_next_fqs, ulong, 0644);
module_param(rcu_kick_kthreads, bool, 0644);
/*
* How long the grace period must be before we start recruiting
......@@ -459,6 +478,28 @@ unsigned long rcu_batches_completed_bh(void)
}
EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);
/*
* Return the number of RCU expedited batches completed thus far for
* debug & stats. Odd numbers mean that a batch is in progress, even
* numbers mean idle. The value returned will thus be roughly double
* the cumulative batches since boot.
*/
unsigned long rcu_exp_batches_completed(void)
{
return rcu_state_p->expedited_sequence;
}
EXPORT_SYMBOL_GPL(rcu_exp_batches_completed);
/*
* Return the number of RCU-sched expedited batches completed thus far
* for debug & stats. Similar to rcu_exp_batches_completed().
*/
unsigned long rcu_exp_batches_completed_sched(void)
{
return rcu_sched_state.expedited_sequence;
}
EXPORT_SYMBOL_GPL(rcu_exp_batches_completed_sched);
/*
* Force a quiescent state.
*/
......@@ -637,7 +678,7 @@ static void rcu_eqs_enter_common(long long oldval, bool user)
idle_task(smp_processor_id());
trace_rcu_dyntick(TPS("Error on entry: not idle task"), oldval, 0);
ftrace_dump(DUMP_ORIG);
rcu_ftrace_dump(DUMP_ORIG);
WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
current->pid, current->comm,
idle->pid, idle->comm); /* must be idle task! */
......@@ -799,7 +840,7 @@ static void rcu_eqs_exit_common(long long oldval, int user)
trace_rcu_dyntick(TPS("Error on exit: not idle task"),
oldval, rdtp->dynticks_nesting);
ftrace_dump(DUMP_ORIG);
rcu_ftrace_dump(DUMP_ORIG);
WARN_ONCE(1, "Current pid: %d comm: %s / Idle pid: %d comm: %s",
current->pid, current->comm,
idle->pid, idle->comm); /* must be idle task! */
......@@ -1224,8 +1265,10 @@ static void rcu_check_gp_kthread_starvation(struct rcu_state *rsp)
rsp->gp_flags,
gp_state_getname(rsp->gp_state), rsp->gp_state,
rsp->gp_kthread ? rsp->gp_kthread->state : ~0);
if (rsp->gp_kthread)
if (rsp->gp_kthread) {
sched_show_task(rsp->gp_kthread);
wake_up_process(rsp->gp_kthread);
}
}
}
......@@ -1249,6 +1292,25 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
}
}
/*
* If too much time has passed in the current grace period, and if
* so configured, go kick the relevant kthreads.
*/
static void rcu_stall_kick_kthreads(struct rcu_state *rsp)
{
unsigned long j;
if (!rcu_kick_kthreads)
return;
j = READ_ONCE(rsp->jiffies_kick_kthreads);
if (time_after(jiffies, j) && rsp->gp_kthread) {
WARN_ONCE(1, "Kicking %s grace-period kthread\n", rsp->name);
rcu_ftrace_dump(DUMP_ALL);
wake_up_process(rsp->gp_kthread);
WRITE_ONCE(rsp->jiffies_kick_kthreads, j + HZ);
}
}
static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
{
int cpu;
......@@ -1260,6 +1322,11 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
struct rcu_node *rnp = rcu_get_root(rsp);
long totqlen = 0;
/* Kick and suppress, if so configured. */
rcu_stall_kick_kthreads(rsp);
if (rcu_cpu_stall_suppress)
return;
/* Only let one CPU complain about others per time interval. */
raw_spin_lock_irqsave_rcu_node(rnp, flags);
......@@ -1333,6 +1400,11 @@ static void print_cpu_stall(struct rcu_state *rsp)
struct rcu_node *rnp = rcu_get_root(rsp);
long totqlen = 0;
/* Kick and suppress, if so configured. */
rcu_stall_kick_kthreads(rsp);
if (rcu_cpu_stall_suppress)
return;
/*
* OK, time to rat on ourselves...
* See Documentation/RCU/stallwarn.txt for info on how to debug
......@@ -1377,8 +1449,10 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
unsigned long js;
struct rcu_node *rnp;
if (rcu_cpu_stall_suppress || !rcu_gp_in_progress(rsp))
if ((rcu_cpu_stall_suppress && !rcu_kick_kthreads) ||
!rcu_gp_in_progress(rsp))
return;
rcu_stall_kick_kthreads(rsp);
j = jiffies;
/*
......@@ -2117,8 +2191,11 @@ static int __noreturn rcu_gp_kthread(void *arg)
}
ret = 0;
for (;;) {
if (!ret)
if (!ret) {
rsp->jiffies_force_qs = jiffies + j;
WRITE_ONCE(rsp->jiffies_kick_kthreads,
jiffies + 3 * j);
}
trace_rcu_grace_period(rsp->name,
READ_ONCE(rsp->gpnum),
TPS("fqswait"));
......@@ -2144,6 +2221,15 @@ static int __noreturn rcu_gp_kthread(void *arg)
TPS("fqsend"));
cond_resched_rcu_qs();
WRITE_ONCE(rsp->gp_activity, jiffies);
ret = 0; /* Force full wait till next FQS. */
j = jiffies_till_next_fqs;
if (j > HZ) {
j = HZ;
jiffies_till_next_fqs = HZ;
} else if (j < 1) {
j = 1;
jiffies_till_next_fqs = 1;
}
} else {
/* Deal with stray signal. */
cond_resched_rcu_qs();
......@@ -2152,14 +2238,12 @@ static int __noreturn rcu_gp_kthread(void *arg)
trace_rcu_grace_period(rsp->name,
READ_ONCE(rsp->gpnum),
TPS("fqswaitsig"));
}
j = jiffies_till_next_fqs;
if (j > HZ) {
j = HZ;
jiffies_till_next_fqs = HZ;
} else if (j < 1) {
ret = 1; /* Keep old FQS timing. */
j = jiffies;
if (time_after(jiffies, rsp->jiffies_force_qs))
j = 1;
jiffies_till_next_fqs = 1;
else
j = rsp->jiffies_force_qs - j;
}
}
......@@ -3376,8 +3460,12 @@ static void rcu_exp_gp_seq_end(struct rcu_state *rsp)
}
static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp)
{
unsigned long s;
smp_mb(); /* Caller's modifications seen first by other CPUs. */
return rcu_seq_snap(&rsp->expedited_sequence);
s = rcu_seq_snap(&rsp->expedited_sequence);
trace_rcu_exp_grace_period(rsp->name, s, TPS("snap"));
return s;
}
static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s)
{
......@@ -3469,7 +3557,7 @@ static void __maybe_unused sync_exp_reset_tree(struct rcu_state *rsp)
* for the current expedited grace period. Works only for preemptible
* RCU -- other RCU implementation use other means.
*
* Caller must hold the root rcu_node's exp_funnel_mutex.
* Caller must hold the rcu_state's exp_mutex.
*/
static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
{
......@@ -3485,8 +3573,8 @@ static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
* recursively up the tree. (Calm down, calm down, we do the recursion
* iteratively!)
*
* Caller must hold the root rcu_node's exp_funnel_mutex and the
* specified rcu_node structure's ->lock.
* Caller must hold the rcu_state's exp_mutex and the specified rcu_node
* structure's ->lock.
*/
static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
bool wake, unsigned long flags)
......@@ -3523,7 +3611,7 @@ static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
* Report expedited quiescent state for specified node. This is a
* lock-acquisition wrapper function for __rcu_report_exp_rnp().
*
* Caller must hold the root rcu_node's exp_funnel_mutex.
* Caller must hold the rcu_state's exp_mutex.
*/
static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
struct rcu_node *rnp, bool wake)
......@@ -3536,8 +3624,8 @@ static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
/*
* Report expedited quiescent state for multiple CPUs, all covered by the
* specified leaf rcu_node structure. Caller must hold the root
* rcu_node's exp_funnel_mutex.
* specified leaf rcu_node structure. Caller must hold the rcu_state's
* exp_mutex.
*/
static void rcu_report_exp_cpu_mult(struct rcu_state *rsp, struct rcu_node *rnp,
unsigned long mask, bool wake)
......@@ -3555,7 +3643,6 @@ static void rcu_report_exp_cpu_mult(struct rcu_state *rsp, struct rcu_node *rnp,
/*
* Report expedited quiescent state for specified rcu_data (CPU).
* Caller must hold the root rcu_node's exp_funnel_mutex.
*/
static void rcu_report_exp_rdp(struct rcu_state *rsp, struct rcu_data *rdp,
bool wake)
......@@ -3564,15 +3651,11 @@ static void rcu_report_exp_rdp(struct rcu_state *rsp, struct rcu_data *rdp,
}
/* Common code for synchronize_{rcu,sched}_expedited() work-done checking. */
static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp,
struct rcu_data *rdp,
atomic_long_t *stat, unsigned long s)
static bool sync_exp_work_done(struct rcu_state *rsp, atomic_long_t *stat,
unsigned long s)
{
if (rcu_exp_gp_seq_done(rsp, s)) {
if (rnp)
mutex_unlock(&rnp->exp_funnel_mutex);
else if (rdp)
mutex_unlock(&rdp->exp_funnel_mutex);
trace_rcu_exp_grace_period(rsp->name, s, TPS("done"));
/* Ensure test happens before caller kfree(). */
smp_mb__before_atomic(); /* ^^^ */
atomic_long_inc(stat);
......@@ -3582,59 +3665,65 @@ static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp,
}
/*
* Funnel-lock acquisition for expedited grace periods. Returns a
* pointer to the root rcu_node structure, or NULL if some other
* task did the expedited grace period for us.
* Funnel-lock acquisition for expedited grace periods. Returns true
* if some other task completed an expedited grace period that this task
* can piggy-back on, and with no mutex held. Otherwise, returns false
* with the mutex held, indicating that the caller must actually do the
* expedited grace period.
*/
static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
{
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
struct rcu_node *rnp0;
struct rcu_node *rnp1 = NULL;
struct rcu_node *rnp = rdp->mynode;
struct rcu_node *rnp_root = rcu_get_root(rsp);
/* Low-contention fastpath. */
if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
(rnp == rnp_root ||
ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
!mutex_is_locked(&rsp->exp_mutex) &&
mutex_trylock(&rsp->exp_mutex))
goto fastpath;
/*
* First try directly acquiring the root lock in order to reduce
* latency in the common case where expedited grace periods are
* rare. We check mutex_is_locked() to avoid pathological levels of
* memory contention on ->exp_funnel_mutex in the heavy-load case.
* Each pass through the following loop works its way up
* the rcu_node tree, returning if others have done the work or
* otherwise falls through to acquire rsp->exp_mutex. The mapping
* from CPU to rcu_node structure can be inexact, as it is just
* promoting locality and is not strictly needed for correctness.
*/
rnp0 = rcu_get_root(rsp);
if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) {
if (mutex_trylock(&rnp0->exp_funnel_mutex)) {
if (sync_exp_work_done(rsp, rnp0, NULL,
&rdp->expedited_workdone0, s))
return NULL;
return rnp0;
for (; rnp != NULL; rnp = rnp->parent) {
if (sync_exp_work_done(rsp, &rdp->exp_workdone1, s))
return true;
/* Work not done, either wait here or go up. */
spin_lock(&rnp->exp_lock);
if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
/* Someone else doing GP, so wait for them. */
spin_unlock(&rnp->exp_lock);
trace_rcu_exp_funnel_lock(rsp->name, rnp->level,
rnp->grplo, rnp->grphi,
TPS("wait"));
wait_event(rnp->exp_wq[(s >> 1) & 0x3],
sync_exp_work_done(rsp,
&rdp->exp_workdone2, s));
return true;
}
rnp->exp_seq_rq = s; /* Followers can wait on us. */
spin_unlock(&rnp->exp_lock);
trace_rcu_exp_funnel_lock(rsp->name, rnp->level, rnp->grplo,
rnp->grphi, TPS("nxtlvl"));
}
/*
* Each pass through the following loop works its way
* up the rcu_node tree, returning if others have done the
* work or otherwise falls through holding the root rnp's
* ->exp_funnel_mutex. The mapping from CPU to rcu_node structure
* can be inexact, as it is just promoting locality and is not
* strictly needed for correctness.
*/
if (sync_exp_work_done(rsp, NULL, NULL, &rdp->expedited_workdone1, s))
return NULL;
mutex_lock(&rdp->exp_funnel_mutex);
rnp0 = rdp->mynode;
for (; rnp0 != NULL; rnp0 = rnp0->parent) {
if (sync_exp_work_done(rsp, rnp1, rdp,
&rdp->expedited_workdone2, s))
return NULL;
mutex_lock(&rnp0->exp_funnel_mutex);
if (rnp1)
mutex_unlock(&rnp1->exp_funnel_mutex);
else
mutex_unlock(&rdp->exp_funnel_mutex);
rnp1 = rnp0;
mutex_lock(&rsp->exp_mutex);
fastpath:
if (sync_exp_work_done(rsp, &rdp->exp_workdone3, s)) {
mutex_unlock(&rsp->exp_mutex);
return true;
}
if (sync_exp_work_done(rsp, rnp1, rdp,
&rdp->expedited_workdone3, s))
return NULL;
return rnp1;
rcu_exp_gp_seq_start(rsp);
trace_rcu_exp_grace_period(rsp->name, s, TPS("start"));
return false;
}
/* Invoked on each online non-idle CPU for expedited quiescent state. */
......@@ -3649,6 +3738,11 @@ static void sync_sched_exp_handler(void *data)
if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
__this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))
return;
if (rcu_is_cpu_rrupt_from_idle()) {
rcu_report_exp_rdp(&rcu_sched_state,
this_cpu_ptr(&rcu_sched_data), true);
return;
}
__this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, true);
resched_cpu(smp_processor_id());
}
......@@ -3773,7 +3867,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
rsp->name);
ndetected = 0;
rcu_for_each_leaf_node(rsp, rnp) {
ndetected = rcu_print_task_exp_stall(rnp);
ndetected += rcu_print_task_exp_stall(rnp);
mask = 1;
for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
struct rcu_data *rdp;
......@@ -3783,7 +3877,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
ndetected++;
rdp = per_cpu_ptr(rsp->rda, cpu);
pr_cont(" %d-%c%c%c", cpu,
"O."[cpu_online(cpu)],
"O."[!!cpu_online(cpu)],
"o."[!!(rdp->grpmask & rnp->expmaskinit)],
"N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
}
......@@ -3792,7 +3886,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
jiffies - jiffies_start, rsp->expedited_sequence,
rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
if (!ndetected) {
if (ndetected) {
pr_err("blocking rcu_node structures:");
rcu_for_each_node_breadth_first(rsp, rnp) {
if (rnp == rnp_root)
......@@ -3818,6 +3912,41 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
}
}
/*
* Wait for the current expedited grace period to complete, and then
* wake up everyone who piggybacked on the just-completed expedited
* grace period. Also update all the ->exp_seq_rq counters as needed
* in order to avoid counter-wrap problems.
*/
static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s)
{
struct rcu_node *rnp;
synchronize_sched_expedited_wait(rsp);
rcu_exp_gp_seq_end(rsp);
trace_rcu_exp_grace_period(rsp->name, s, TPS("end"));
/*
* Switch over to wakeup mode, allowing the next GP, but -only- the
* next GP, to proceed.
*/
mutex_lock(&rsp->exp_wake_mutex);
mutex_unlock(&rsp->exp_mutex);
rcu_for_each_node_breadth_first(rsp, rnp) {
if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
spin_lock(&rnp->exp_lock);
/* Recheck, avoid hang in case someone just arrived. */
if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
rnp->exp_seq_rq = s;
spin_unlock(&rnp->exp_lock);
}
wake_up_all(&rnp->exp_wq[(rsp->expedited_sequence >> 1) & 0x3]);
}
trace_rcu_exp_grace_period(rsp->name, s, TPS("endwake"));
mutex_unlock(&rsp->exp_wake_mutex);
}
/**
* synchronize_sched_expedited - Brute-force RCU-sched grace period
*
......@@ -3837,7 +3966,6 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
void synchronize_sched_expedited(void)
{
unsigned long s;
struct rcu_node *rnp;
struct rcu_state *rsp = &rcu_sched_state;
/* If only one CPU, this is automatically a grace period. */
......@@ -3852,17 +3980,14 @@ void synchronize_sched_expedited(void)
/* Take a snapshot of the sequence number. */
s = rcu_exp_gp_seq_snap(rsp);
rnp = exp_funnel_lock(rsp, s);
if (rnp == NULL)
if (exp_funnel_lock(rsp, s))
return; /* Someone else did our work for us. */
rcu_exp_gp_seq_start(rsp);
/* Initialize the rcu_node tree in preparation for the wait. */
sync_rcu_exp_select_cpus(rsp, sync_sched_exp_handler);
synchronize_sched_expedited_wait(rsp);
rcu_exp_gp_seq_end(rsp);
mutex_unlock(&rnp->exp_funnel_mutex);
/* Wait and clean up, including waking everyone. */
rcu_exp_wait_wake(rsp, s);
}
EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
......@@ -4162,7 +4287,6 @@ rcu_boot_init_percpu_data(int cpu, struct rcu_state *rsp)
WARN_ON_ONCE(atomic_read(&rdp->dynticks->dynticks) != 1);
rdp->cpu = cpu;
rdp->rsp = rsp;
mutex_init(&rdp->exp_funnel_mutex);
rcu_boot_init_nocb_percpu_data(rdp);
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
......@@ -4420,10 +4544,8 @@ static void __init rcu_init_one(struct rcu_state *rsp)
{
static const char * const buf[] = RCU_NODE_NAME_INIT;
static const char * const fqs[] = RCU_FQS_NAME_INIT;
static const char * const exp[] = RCU_EXP_NAME_INIT;
static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
static struct lock_class_key rcu_fqs_class[RCU_NUM_LVLS];
static struct lock_class_key rcu_exp_class[RCU_NUM_LVLS];
static u8 fl_mask = 0x1;
int levelcnt[RCU_NUM_LVLS]; /* # nodes in each level. */
......@@ -4482,9 +4604,11 @@ static void __init rcu_init_one(struct rcu_state *rsp)
rnp->level = i;
INIT_LIST_HEAD(&rnp->blkd_tasks);
rcu_init_one_nocb(rnp);
mutex_init(&rnp->exp_funnel_mutex);
lockdep_set_class_and_name(&rnp->exp_funnel_mutex,
&rcu_exp_class[i], exp[i]);
init_waitqueue_head(&rnp->exp_wq[0]);
init_waitqueue_head(&rnp->exp_wq[1]);
init_waitqueue_head(&rnp->exp_wq[2]);
init_waitqueue_head(&rnp->exp_wq[3]);
spin_lock_init(&rnp->exp_lock);
}
}
......
kernel/rcu/tree.h
View file @ a3871bd4
......@@ -70,7 +70,6 @@
# define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 }
# define RCU_NODE_NAME_INIT { "rcu_node_0" }
# define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" }
# define RCU_EXP_NAME_INIT { "rcu_node_exp_0" }
#elif NR_CPUS <= RCU_FANOUT_2
# define RCU_NUM_LVLS 2
# define NUM_RCU_LVL_0 1
......@@ -79,7 +78,6 @@
# define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
# define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" }
# define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" }
# define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" }
#elif NR_CPUS <= RCU_FANOUT_3
# define RCU_NUM_LVLS 3
# define NUM_RCU_LVL_0 1
......@@ -89,7 +87,6 @@
# define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
# define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
# define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
# define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
#elif NR_CPUS <= RCU_FANOUT_4
# define RCU_NUM_LVLS 4
# define NUM_RCU_LVL_0 1
......@@ -100,7 +97,6 @@
# define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
# define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
# define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
# define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
#else
# error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
#endif /* #if (NR_CPUS) <= RCU_FANOUT_1 */
......@@ -252,7 +248,9 @@ struct rcu_node {
/* Counts of upcoming no-CB GP requests. */
raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
struct mutex exp_funnel_mutex ____cacheline_internodealigned_in_smp;
spinlock_t exp_lock ____cacheline_internodealigned_in_smp;
unsigned long exp_seq_rq;
wait_queue_head_t exp_wq[4];
} ____cacheline_internodealigned_in_smp;
/*
......@@ -387,11 +385,9 @@ struct rcu_data {
#ifdef CONFIG_RCU_FAST_NO_HZ
struct rcu_head oom_head;
#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
struct mutex exp_funnel_mutex;
atomic_long_t expedited_workdone0; /* # done by others #0. */
atomic_long_t expedited_workdone1; /* # done by others #1. */
atomic_long_t expedited_workdone2; /* # done by others #2. */
atomic_long_t expedited_workdone3; /* # done by others #3. */
atomic_long_t exp_workdone1; /* # done by others #1. */
atomic_long_t exp_workdone2; /* # done by others #2. */
atomic_long_t exp_workdone3; /* # done by others #3. */
/* 7) Callback offloading. */
#ifdef CONFIG_RCU_NOCB_CPU
......@@ -505,6 +501,8 @@ struct rcu_state {
/* _rcu_barrier(). */
/* End of fields guarded by barrier_mutex. */
struct mutex exp_mutex; /* Serialize expedited GP. */
struct mutex exp_wake_mutex; /* Serialize wakeup. */
unsigned long expedited_sequence; /* Take a ticket. */
atomic_long_t expedited_normal; /* # fallbacks to normal. */
atomic_t expedited_need_qs; /* # CPUs left to check in. */
......@@ -513,6 +511,8 @@ struct rcu_state {
unsigned long jiffies_force_qs; /* Time at which to invoke */
/* force_quiescent_state(). */
unsigned long jiffies_kick_kthreads; /* Time at which to kick */
/* kthreads, if configured. */
unsigned long n_force_qs; /* Number of calls to */
/* force_quiescent_state(). */
unsigned long n_force_qs_lh; /* ~Number of calls leaving */
......
kernel/rcu/tree_plugin.h
View file @ a3871bd4
......@@ -722,18 +722,22 @@ static void sync_rcu_exp_handler(void *info)
* synchronize_rcu_expedited - Brute-force RCU grace period
*
* Wait for an RCU-preempt grace period, but expedite it. The basic
* idea is to invoke synchronize_sched_expedited() to push all the tasks to
* the ->blkd_tasks lists and wait for this list to drain. This consumes
* significant time on all CPUs and is unfriendly to real-time workloads,
* so is thus not recommended for any sort of common-case code.
* In fact, if you are using synchronize_rcu_expedited() in a loop,
* please restructure your code to batch your updates, and then Use a
* single synchronize_rcu() instead.
* idea is to IPI all non-idle non-nohz online CPUs. The IPI handler
* checks whether the CPU is in an RCU-preempt critical section, and
* if so, it sets a flag that causes the outermost rcu_read_unlock()
* to report the quiescent state. On the other hand, if the CPU is
* not in an RCU read-side critical section, the IPI handler reports
* the quiescent state immediately.
*
* Although this is a greate improvement over previous expedited
* implementations, it is still unfriendly to real-time workloads, so is
* thus not recommended for any sort of common-case code. In fact, if
* you are using synchronize_rcu_expedited() in a loop, please restructure
* your code to batch your updates, and then Use a single synchronize_rcu()
* instead.
*/
void synchronize_rcu_expedited(void)
{
struct rcu_node *rnp;
struct rcu_node *rnp_unlock;
struct rcu_state *rsp = rcu_state_p;
unsigned long s;
......@@ -744,23 +748,14 @@ void synchronize_rcu_expedited(void)
}
s = rcu_exp_gp_seq_snap(rsp);
rnp_unlock = exp_funnel_lock(rsp, s);
if (rnp_unlock == NULL)
if (exp_funnel_lock(rsp, s))
return; /* Someone else did our work for us. */
rcu_exp_gp_seq_start(rsp);
/* Initialize the rcu_node tree in preparation for the wait. */
sync_rcu_exp_select_cpus(rsp, sync_rcu_exp_handler);
/* Wait for snapshotted ->blkd_tasks lists to drain. */
rnp = rcu_get_root(rsp);
synchronize_sched_expedited_wait(rsp);
/* Clean up and exit. */
rcu_exp_gp_seq_end(rsp);
mutex_unlock(&rnp_unlock->exp_funnel_mutex);
/* Wait for ->blkd_tasks lists to drain, then wake everyone up. */
rcu_exp_wait_wake(rsp, s);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
......
kernel/rcu/tree_trace.c
View file @ a3871bd4
......@@ -185,17 +185,16 @@ static int show_rcuexp(struct seq_file *m, void *v)
int cpu;
struct rcu_state *rsp = (struct rcu_state *)m->private;
struct rcu_data *rdp;
unsigned long s0 = 0, s1 = 0, s2 = 0, s3 = 0;
unsigned long s1 = 0, s2 = 0, s3 = 0;
for_each_possible_cpu(cpu) {
rdp = per_cpu_ptr(rsp->rda, cpu);
s0 += atomic_long_read(&rdp->expedited_workdone0);
s1 += atomic_long_read(&rdp->expedited_workdone1);
s2 += atomic_long_read(&rdp->expedited_workdone2);
s3 += atomic_long_read(&rdp->expedited_workdone3);
s1 += atomic_long_read(&rdp->exp_workdone1);
s2 += atomic_long_read(&rdp->exp_workdone2);
s3 += atomic_long_read(&rdp->exp_workdone3);
}
seq_printf(m, "s=%lu wd0=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",
rsp->expedited_sequence, s0, s1, s2, s3,
seq_printf(m, "s=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",
rsp->expedited_sequence, s1, s2, s3,
atomic_long_read(&rsp->expedited_normal),
atomic_read(&rsp->expedited_need_qs),
rsp->expedited_sequence / 2);
......
kernel/rcu/update.c
View file @ a3871bd4
......@@ -67,7 +67,7 @@ static int rcu_normal_after_boot;
module_param(rcu_normal_after_boot, int, 0);
#endif /* #ifndef CONFIG_TINY_RCU */
#if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
#ifdef CONFIG_DEBUG_LOCK_ALLOC
/**
* rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
*
......@@ -111,7 +111,7 @@ int rcu_read_lock_sched_held(void)
return 0;
if (debug_locks)
lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
return lockdep_opinion || !preemptible();
}
EXPORT_SYMBOL(rcu_read_lock_sched_held);
#endif
......
kernel/torture.c
View file @ a3871bd4
......@@ -451,6 +451,7 @@ static int torture_shutdown(void *arg)
torture_shutdown_hook();
else
VERBOSE_TOROUT_STRING("No torture_shutdown_hook(), skipping.");
ftrace_dump(DUMP_ALL);
kernel_power_off(); /* Shut down the system. */
return 0;
}
......@@ -602,8 +603,9 @@ bool torture_init_begin(char *ttype, bool v, int *runnable)
{
mutex_lock(&fullstop_mutex);
if (torture_type != NULL) {
pr_alert("torture_init_begin: refusing %s init: %s running",
pr_alert("torture_init_begin: Refusing %s init: %s running.\n",
ttype, torture_type);
pr_alert("torture_init_begin: One torture test at a time!\n");
mutex_unlock(&fullstop_mutex);
return false;
}
......
lib/Kconfig.debug
View file @ a3871bd4
......@@ -1289,6 +1289,39 @@ config TORTURE_TEST
tristate
default n
config RCU_PERF_TEST
tristate "performance tests for RCU"
depends on DEBUG_KERNEL
select TORTURE_TEST
select SRCU
select TASKS_RCU
default n
help
This option provides a kernel module that runs performance
tests on the RCU infrastructure. The kernel module may be built
after the fact on the running kernel to be tested, if desired.
Say Y here if you want RCU performance tests to be built into
the kernel.
Say M if you want the RCU performance tests to build as a module.
Say N if you are unsure.
config RCU_PERF_TEST_RUNNABLE
bool "performance tests for RCU runnable by default"
depends on RCU_PERF_TEST = y
default n
help
This option provides a way to build the RCU performance tests
directly into the kernel without them starting up at boot time.
You can use /sys/module to manually override this setting.
This /proc file is available only when the RCU performance
tests have been built into the kernel.
Say Y here if you want the RCU performance tests to start during
boot (you probably don't).
Say N here if you want the RCU performance tests to start only
after being manually enabled via /sys/module.
config RCU_TORTURE_TEST
tristate "torture tests for RCU"
depends on DEBUG_KERNEL
......
tools/testing/selftests/rcutorture/bin/jitter.sh 0 → 100755
View file @ a3871bd4
#!/bin/bash
#
# Alternate sleeping and spinning on randomly selected CPUs. The purpose
# of this script is to inflict random OS jitter on a concurrently running
# test.
#
# Usage: jitter.sh me duration [ sleepmax [ spinmax ] ]
#
# me: Random-number-generator seed salt.
# duration: Time to run in seconds.
# sleepmax: Maximum microseconds to sleep, defaults to one second.
# spinmax: Maximum microseconds to spin, defaults to one millisecond.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, you can access it online at
# http://www.gnu.org/licenses/gpl-2.0.html.
#
# Copyright (C) IBM Corporation, 2016
#
# Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
me=$(($1 * 1000))
duration=$2
sleepmax=${3-1000000}
spinmax=${4-1000}
n=1
starttime=`awk 'BEGIN { print systime(); }' < /dev/null`
while :
do
# Check for done.
t=`awk -v s=$starttime 'BEGIN { print systime() - s; }' < /dev/null`
if test "$t" -gt "$duration"
then
exit 0;
fi
# Set affinity to randomly selected CPU
cpus=`ls /sys/devices/system/cpu/*/online |
sed -e 's,/[^/]*$,,' -e 's/^[^0-9]*//' |
grep -v '^0*$'`
cpumask=`awk -v cpus="$cpus" -v me=$me -v n=$n 'BEGIN {
srand(n + me + systime());
ncpus = split(cpus, ca);
curcpu = ca[int(rand() * ncpus + 1)];
mask = lshift(1, curcpu);
if (mask + 0 <= 0)
mask = 1;
printf("%#x\n", mask);
}' < /dev/null`
n=$(($n+1))
if ! taskset -p $cpumask $$ > /dev/null 2>&1
then
echo taskset failure: '"taskset -p ' $cpumask $$ '"'
exit 1
fi
# Sleep a random duration
sleeptime=`awk -v me=$me -v n=$n -v sleepmax=$sleepmax 'BEGIN {
srand(n + me + systime());
printf("%06d", int(rand() * sleepmax));
}' < /dev/null`
n=$(($n+1))
sleep .$sleeptime
# Spin a random duration
limit=`awk -v me=$me -v n=$n -v spinmax=$spinmax 'BEGIN {
srand(n + me + systime());
printf("%06d", int(rand() * spinmax));
}' < /dev/null`
n=$(($n+1))
for i in {1..$limit}
do
echo > /dev/null
done
done
exit 1
tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf-ftrace.sh 0 → 100755
View file @ a3871bd4
#!/bin/bash
#
# Analyze a given results directory for rcuperf performance measurements,
# looking for ftrace data. Exits with 0 if data was found, analyzed, and
# printed. Intended to be invoked from kvm-recheck-rcuperf.sh after
# argument checking.
#
# Usage: kvm-recheck-rcuperf-ftrace.sh resdir
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, you can access it online at
# http://www.gnu.org/licenses/gpl-2.0.html.
#
# Copyright (C) IBM Corporation, 2016
#
# Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
i="$1"
. tools/testing/selftests/rcutorture/bin/functions.sh
if test "`grep -c 'rcu_exp_grace_period.*start' < $i/console.log`" -lt 100
then
exit 10
fi
sed -e 's/^\[[^]]*]//' < $i/console.log |
grep 'us : rcu_exp_grace_period' |
sed -e 's/us : / : /' |
tr -d '\015' |
awk '
$8 == "start" {
if (starttask != "")
nlost++;
starttask = $1;
starttime = $3;
startseq = $7;
}
$8 == "end" {
if (starttask == $1 && startseq == $7) {
curgpdur = $3 - starttime;
gptimes[++n] = curgpdur;
gptaskcnt[starttask]++;
sum += curgpdur;
if (curgpdur > 1000)
print "Long GP " starttime "us to " $3 "us (" curgpdur "us)";
starttask = "";
} else {
# Lost a message or some such, reset.
starttask = "";
nlost++;
}
}
$8 == "done" {
piggybackcnt[$1]++;
}
END {
newNR = asort(gptimes);
if (newNR <= 0) {
print "No ftrace records found???"
exit 10;
}
pct50 = int(newNR * 50 / 100);
if (pct50 < 1)
pct50 = 1;
pct90 = int(newNR * 90 / 100);
if (pct90 < 1)
pct90 = 1;
pct99 = int(newNR * 99 / 100);
if (pct99 < 1)
pct99 = 1;
div = 10 ** int(log(gptimes[pct90]) / log(10) + .5) / 100;
print "Histogram bucket size: " div;
last = gptimes[1] - 10;
count = 0;
for (i = 1; i <= newNR; i++) {
current = div * int(gptimes[i] / div);
if (last == current) {
count++;
} else {
if (count > 0)
print last, count;
count = 1;
last = current;
}
}
if (count > 0)
print last, count;
print "Distribution of grace periods across tasks:";
for (i in gptaskcnt) {
print "\t" i, gptaskcnt[i];
nbatches += gptaskcnt[i];
}
ngps = nbatches;
print "Distribution of piggybacking across tasks:";
for (i in piggybackcnt) {
print "\t" i, piggybackcnt[i];
ngps += piggybackcnt[i];
}
print "Average grace-period duration: " sum / newNR " microseconds";
print "Minimum grace-period duration: " gptimes[1];
print "50th percentile grace-period duration: " gptimes[pct50];
print "90th percentile grace-period duration: " gptimes[pct90];
print "99th percentile grace-period duration: " gptimes[pct99];
print "Maximum grace-period duration: " gptimes[newNR];
print "Grace periods: " ngps + 0 " Batches: " nbatches + 0 " Ratio: " ngps / nbatches " Lost: " nlost + 0;
print "Computed from ftrace data.";
}'
exit 0
tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf.sh 0 → 100755
View file @ a3871bd4
#!/bin/bash
#
# Analyze a given results directory for rcuperf performance measurements.
#
# Usage: kvm-recheck-rcuperf.sh resdir
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, you can access it online at
# http://www.gnu.org/licenses/gpl-2.0.html.
#
# Copyright (C) IBM Corporation, 2016
#
# Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
i="$1"
if test -d $i
then
:
else
echo Unreadable results directory: $i
exit 1
fi
PATH=`pwd`/tools/testing/selftests/rcutorture/bin:$PATH; export PATH
. tools/testing/selftests/rcutorture/bin/functions.sh
if kvm-recheck-rcuperf-ftrace.sh $i
then
# ftrace data was successfully analyzed, call it good!
exit 0
fi
configfile=`echo $i | sed -e 's/^.*\///'`
sed -e 's/^\[[^]]*]//' < $i/console.log |
awk '
/-perf: .* gps: .* batches:/ {
ngps = $9;
nbatches = $11;
}
/-perf: .*writer-duration/ {
gptimes[++n] = $5 / 1000.;
sum += $5 / 1000.;
}
END {
newNR = asort(gptimes);
if (newNR <= 0) {
print "No rcuperf records found???"
exit;
}
pct50 = int(newNR * 50 / 100);
if (pct50 < 1)
pct50 = 1;
pct90 = int(newNR * 90 / 100);
if (pct90 < 1)
pct90 = 1;
pct99 = int(newNR * 99 / 100);
if (pct99 < 1)
pct99 = 1;
div = 10 ** int(log(gptimes[pct90]) / log(10) + .5) / 100;
print "Histogram bucket size: " div;
last = gptimes[1] - 10;
count = 0;
for (i = 1; i <= newNR; i++) {
current = div * int(gptimes[i] / div);
if (last == current) {
count++;
} else {
if (count > 0)
print last, count;
count = 1;
last = current;
}
}
if (count > 0)
print last, count;
print "Average grace-period duration: " sum / newNR " microseconds";
print "Minimum grace-period duration: " gptimes[1];
print "50th percentile grace-period duration: " gptimes[pct50];
print "90th percentile grace-period duration: " gptimes[pct90];
print "99th percentile grace-period duration: " gptimes[pct99];
print "Maximum grace-period duration: " gptimes[newNR];
print "Grace periods: " ngps + 0 " Batches: " nbatches + 0 " Ratio: " ngps / nbatches;
print "Computed from rcuperf printk output.";
}'
tools/testing/selftests/rcutorture/bin/kvm-recheck.sh
View file @ a3871bd4
......@@ -48,7 +48,10 @@ do
cat $i/Make.oldconfig.err
fi
parse-build.sh $i/Make.out $configfile
if test "$TORTURE_SUITE" != rcuperf
then
parse-torture.sh $i/console.log $configfile
fi
parse-console.sh $i/console.log $configfile
if test -r $i/Warnings
then
......
tools/testing/selftests/rcutorture/bin/kvm-test-1-run.sh
View file @ a3871bd4
......@@ -6,7 +6,7 @@
# Execute this in the source tree. Do not run it as a background task
# because qemu does not seem to like that much.
#
# Usage: kvm-test-1-run.sh config builddir resdir minutes qemu-args boot_args
# Usage: kvm-test-1-run.sh config builddir resdir seconds qemu-args boot_args
#
# qemu-args defaults to "-enable-kvm -soundhw pcspk -nographic", along with
# arguments specifying the number of CPUs and other
......@@ -91,25 +91,33 @@ fi
# CONFIG_PCMCIA=n
# CONFIG_CARDBUS=n
# CONFIG_YENTA=n
if kvm-build.sh $config_template $builddir $T
base_resdir=`echo $resdir | sed -e 's/\.[0-9]\+$//'`
if test "$base_resdir" != "$resdir" -a -f $base_resdir/bzImage -a -f $base_resdir/vmlinux
then
# Rerunning previous test, so use that test's kernel.
QEMU="`identify_qemu $base_resdir/vmlinux`"
KERNEL=$base_resdir/bzImage
ln -s $base_resdir/Make*.out $resdir # for kvm-recheck.sh
ln -s $base_resdir/.config $resdir # for kvm-recheck.sh
elif kvm-build.sh $config_template $builddir $T
then
# Had to build a kernel for this test.
QEMU="`identify_qemu $builddir/vmlinux`"
BOOT_IMAGE="`identify_boot_image $QEMU`"
cp $builddir/Make*.out $resdir
cp $builddir/vmlinux $resdir
cp $builddir/.config $resdir
if test -n "$BOOT_IMAGE"
then
cp $builddir/$BOOT_IMAGE $resdir
KERNEL=$resdir/bzImage
else
echo No identifiable boot image, not running KVM, see $resdir.
echo Do the torture scripts know about your architecture?
fi
parse-build.sh $resdir/Make.out $title
if test -f $builddir.wait
then
mv $builddir.wait $builddir.ready
fi
else
# Build failed.
cp $builddir/Make*.out $resdir
cp $builddir/.config $resdir || :
echo Build failed, not running KVM, see $resdir.
......@@ -119,12 +127,15 @@ else
fi
exit 1
fi
if test -f $builddir.wait
then
mv $builddir.wait $builddir.ready
fi
while test -f $builddir.ready
do
sleep 1
done
minutes=$4
seconds=$(($minutes * 60))
seconds=$4
qemu_args=$5
boot_args=$6
......@@ -167,15 +178,26 @@ then
exit 0
fi
echo "NOTE: $QEMU either did not run or was interactive" > $resdir/console.log
echo $QEMU $qemu_args -m 512 -kernel $resdir/bzImage -append \"$qemu_append $boot_args\" > $resdir/qemu-cmd
( $QEMU $qemu_args -m 512 -kernel $resdir/bzImage -append "$qemu_append $boot_args"; echo $? > $resdir/qemu-retval ) &
qemu_pid=$!
echo $QEMU $qemu_args -m 512 -kernel $KERNEL -append \"$qemu_append $boot_args\" > $resdir/qemu-cmd
( $QEMU $qemu_args -m 512 -kernel $KERNEL -append "$qemu_append $boot_args"& echo $! > $resdir/qemu_pid; wait `cat $resdir/qemu_pid`; echo $? > $resdir/qemu-retval ) &
commandcompleted=0
echo Monitoring qemu job at pid $qemu_pid
sleep 10 # Give qemu's pid a chance to reach the file
if test -s "$resdir/qemu_pid"
then
qemu_pid=`cat "$resdir/qemu_pid"`
echo Monitoring qemu job at pid $qemu_pid
else
qemu_pid=""
echo Monitoring qemu job at yet-as-unknown pid
fi
while :
do
if test -z "$qemu_pid" -a -s "$resdir/qemu_pid"
then
qemu_pid=`cat "$resdir/qemu_pid"`
fi
kruntime=`awk 'BEGIN { print systime() - '"$kstarttime"' }' < /dev/null`
if kill -0 $qemu_pid > /dev/null 2>&1
if test -z "$qemu_pid" || kill -0 "$qemu_pid" > /dev/null 2>&1
then
if test $kruntime -ge $seconds
then
......@@ -195,12 +217,16 @@ do
ps -fp $killpid >> $resdir/Warnings 2>&1
fi
else
echo ' ---' `date`: Kernel done
echo ' ---' `date`: "Kernel done"
fi
break
fi
done
if test $commandcompleted -eq 0
if test -z "$qemu_pid" -a -s "$resdir/qemu_pid"
then
qemu_pid=`cat "$resdir/qemu_pid"`
fi
if test $commandcompleted -eq 0 -a -n "$qemu_pid"
then
echo Grace period for qemu job at pid $qemu_pid
while :
......@@ -220,6 +246,9 @@ then
fi
sleep 1
done
elif test -z "$qemu_pid"
then
echo Unknown PID, cannot kill qemu command
fi
parse-torture.sh $resdir/console.log $title
......
tools/testing/selftests/rcutorture/bin/kvm.sh
View file @ a3871bd4
......@@ -34,7 +34,7 @@ T=/tmp/kvm.sh.$$
trap 'rm -rf $T' 0
mkdir $T
dur=30
dur=$((30*60))
dryrun=""
KVM="`pwd`/tools/testing/selftests/rcutorture"; export KVM
PATH=${KVM}/bin:$PATH; export PATH
......@@ -48,6 +48,7 @@ resdir=""
configs=""
cpus=0
ds=`date +%Y.%m.%d-%H:%M:%S`
jitter=0
. functions.sh
......@@ -63,6 +64,7 @@ usage () {
echo " --dryrun sched|script"
echo " --duration minutes"
echo " --interactive"
echo " --jitter N [ maxsleep (us) [ maxspin (us) ] ]"
echo " --kmake-arg kernel-make-arguments"
echo " --mac nn:nn:nn:nn:nn:nn"
echo " --no-initrd"
......@@ -116,12 +118,17 @@ do
;;
--duration)
checkarg --duration "(minutes)" $# "$2" '^[0-9]*$' '^error'
dur=$2
dur=$(($2*60))
shift
;;
--interactive)
TORTURE_QEMU_INTERACTIVE=1; export TORTURE_QEMU_INTERACTIVE
;;
--jitter)
checkarg --jitter "(# threads [ sleep [ spin ] ])" $# "$2" '^-\{,1\}[0-9]\+\( \+[0-9]\+\)\{,2\} *$' '^error$'
jitter="$2"
shift
;;
--kmake-arg)
checkarg --kmake-arg "(kernel make arguments)" $# "$2" '.*' '^error$'
TORTURE_KMAKE_ARG="$2"
......@@ -156,7 +163,7 @@ do
shift
;;
--torture)
checkarg --torture "(suite name)" "$#" "$2" '^\(lock\|rcu\)$' '^--'
checkarg --torture "(suite name)" "$#" "$2" '^\(lock\|rcu\|rcuperf\)$' '^--'
TORTURE_SUITE=$2
shift
;;
......@@ -299,6 +306,7 @@ awk < $T/cfgcpu.pack \
-v CONFIGDIR="$CONFIGFRAG/" \
-v KVM="$KVM" \
-v ncpus=$cpus \
-v jitter="$jitter" \
-v rd=$resdir/$ds/ \
-v dur=$dur \
-v TORTURE_QEMU_ARG="$TORTURE_QEMU_ARG" \
......@@ -359,6 +367,16 @@ function dump(first, pastlast, batchnum)
print "\techo ----", cfr[j], cpusr[j] ovf ": Starting kernel. `date` >> " rd "/log";
print "fi"
}
njitter = 0;
split(jitter, ja);
if (ja[1] == -1 && ncpus == 0)
njitter = 1;
else if (ja[1] == -1)
njitter = ncpus;
else
njitter = ja[1];
for (j = 0; j < njitter; j++)
print "jitter.sh " j " " dur " " ja[2] " " ja[3] "&"
print "wait"
print "if test -z \"$TORTURE_BUILDONLY\""
print "then"
......
tools/testing/selftests/rcutorture/configs/rcu/TREE04
View file @ a3871bd4
......@@ -14,7 +14,7 @@ CONFIG_HOTPLUG_CPU=n
CONFIG_SUSPEND=n
CONFIG_HIBERNATION=n
CONFIG_RCU_FANOUT=4
CONFIG_RCU_FANOUT_LEAF=4
CONFIG_RCU_FANOUT_LEAF=3
CONFIG_RCU_NOCB_CPU=n
CONFIG_DEBUG_LOCK_ALLOC=n
CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
......
tools/testing/selftests/rcutorture/configs/rcu/TREE04.boot
View file @ a3871bd4
rcutorture.torture_type=rcu_bh
rcutorture.torture_type=rcu_bh rcutree.rcu_fanout_leaf=4
tools/testing/selftests/rcutorture/configs/rcuperf/CFcommon 0 → 100644
View file @ a3871bd4
CONFIG_RCU_PERF_TEST=y
CONFIG_PRINTK_TIME=y
tools/testing/selftests/rcutorture/configs/rcuperf/TREE 0 → 100644
View file @ a3871bd4
CONFIG_SMP=y
CONFIG_PREEMPT_NONE=n
CONFIG_PREEMPT_VOLUNTARY=n
CONFIG_PREEMPT=y
#CHECK#CONFIG_PREEMPT_RCU=y
CONFIG_HZ_PERIODIC=n
CONFIG_NO_HZ_IDLE=y
CONFIG_NO_HZ_FULL=n
CONFIG_RCU_FAST_NO_HZ=n
CONFIG_RCU_TRACE=n
CONFIG_HOTPLUG_CPU=n
CONFIG_SUSPEND=n
CONFIG_HIBERNATION=n
CONFIG_RCU_NOCB_CPU=n
CONFIG_DEBUG_LOCK_ALLOC=n
CONFIG_PROVE_LOCKING=n
CONFIG_RCU_BOOST=n
CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
CONFIG_RCU_EXPERT=y
CONFIG_RCU_TRACE=y
tools/testing/selftests/rcutorture/configs/rcuperf/TREE54 0 → 100644
View file @ a3871bd4
CONFIG_SMP=y
CONFIG_NR_CPUS=54
CONFIG_PREEMPT_NONE=n
CONFIG_PREEMPT_VOLUNTARY=n
CONFIG_PREEMPT=y
#CHECK#CONFIG_PREEMPT_RCU=y
CONFIG_HZ_PERIODIC=n
CONFIG_NO_HZ_IDLE=y
CONFIG_NO_HZ_FULL=n
CONFIG_RCU_FAST_NO_HZ=n
CONFIG_RCU_TRACE=n
CONFIG_HOTPLUG_CPU=n
CONFIG_SUSPEND=n
CONFIG_HIBERNATION=n
CONFIG_RCU_FANOUT=3
CONFIG_RCU_FANOUT_LEAF=2
CONFIG_RCU_NOCB_CPU=n
CONFIG_DEBUG_LOCK_ALLOC=n
CONFIG_PROVE_LOCKING=n
CONFIG_RCU_BOOST=n
CONFIG_DEBUG_OBJECTS_RCU_HEAD=n
CONFIG_RCU_EXPERT=y
CONFIG_RCU_TRACE=y
tools/testing/selftests/rcutorture/configs/rcuperf/ver_functions.sh 0 → 100644
View file @ a3871bd4
#!/bin/bash
#
# Torture-suite-dependent shell functions for the rest of the scripts.
#
# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, you can access it online at
# http://www.gnu.org/licenses/gpl-2.0.html.
#
# Copyright (C) IBM Corporation, 2015
#
# Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
# rcuperf_param_nreaders bootparam-string
#
# Adds nreaders rcuperf module parameter if not already specified.
rcuperf_param_nreaders () {
if ! echo "$1" | grep -q "rcuperf.nreaders"
then
echo rcuperf.nreaders=-1
fi
}
# rcuperf_param_nwriters bootparam-string
#
# Adds nwriters rcuperf module parameter if not already specified.
rcuperf_param_nwriters () {
if ! echo "$1" | grep -q "rcuperf.nwriters"
then
echo rcuperf.nwriters=-1
fi
}
# per_version_boot_params bootparam-string config-file seconds
#
# Adds per-version torture-module parameters to kernels supporting them.
per_version_boot_params () {
echo $1 `rcuperf_param_nreaders "$1"` \
`rcuperf_param_nwriters "$1"` \
rcuperf.perf_runnable=1 \
rcuperf.shutdown=1 \
rcuperf.verbose=1
}
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