Commit b26bf6ab authored by Rafael J. Wysocki's avatar Rafael J. Wysocki

cpuidle: New timer events oriented governor for tickless systems

The venerable menu governor does some things that are quite
questionable in my view.

First, it includes timer wakeups in the pattern detection data and
mixes them up with wakeups from other sources which in some cases
causes it to expect what essentially would be a timer wakeup in a
time frame in which no timer wakeups are possible (because it knows
the time until the next timer event and that is later than the
expected wakeup time).

Second, it uses the extra exit latency limit based on the predicted
idle duration and depending on the number of tasks waiting on I/O,
even though those tasks may run on a different CPU when they are
woken up.  Moreover, the time ranges used by it for the sleep length
correction factors depend on whether or not there are tasks waiting
on I/O, which again doesn't imply anything in particular, and they
are not correlated to the list of available idle states in any way
whatever.

Also, the pattern detection code in menu may end up considering
values that are too large to matter at all, in which cases running
it is a waste of time.

A major rework of the menu governor would be required to address
these issues and the performance of at least some workloads (tuned
specifically to the current behavior of the menu governor) is likely
to suffer from that.  It is thus better to introduce an entirely new
governor without them and let everybody use the governor that works
better with their actual workloads.

The new governor introduced here, the timer events oriented (TEO)
governor, uses the same basic strategy as menu: it always tries to
find the deepest idle state that can be used in the given conditions.
However, it applies a different approach to that problem.

First, it doesn't use "correction factors" for the time till the
closest timer, but instead it tries to correlate the measured idle
duration values with the available idle states and use that
information to pick up the idle state that is most likely to "match"
the upcoming CPU idle interval.

Second, it doesn't take the number of "I/O waiters" into account at
all and the pattern detection code in it avoids taking timer wakeups
into account.  It also only uses idle duration values less than the
current time till the closest timer (with the tick excluded) for that
purpose.
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: default avatarDaniel Lezcano <daniel.lezcano@linaro.org>
parent 1c7fc5cb
......@@ -155,14 +155,14 @@ governor uses that information depends on what algorithm is implemented by it
and that is the primary reason for having more than one governor in the
``CPUIdle`` subsystem.
There are two ``CPUIdle`` governors available, ``menu`` and ``ladder``. Which
of them is used depends on the configuration of the kernel and in particular on
whether or not the scheduler tick can be `stopped by the idle
loop <idle-cpus-and-tick_>`_. It is possible to change the governor at run time
if the ``cpuidle_sysfs_switch`` command line parameter has been passed to the
kernel, but that is not safe in general, so it should not be done on production
systems (that may change in the future, though). The name of the ``CPUIdle``
governor currently used by the kernel can be read from the
There are three ``CPUIdle`` governors available, ``menu``, `TEO <teo-gov_>`_
and ``ladder``. Which of them is used by default depends on the configuration
of the kernel and in particular on whether or not the scheduler tick can be
`stopped by the idle loop <idle-cpus-and-tick_>`_. It is possible to change the
governor at run time if the ``cpuidle_sysfs_switch`` command line parameter has
been passed to the kernel, but that is not safe in general, so it should not be
done on production systems (that may change in the future, though). The name of
the ``CPUIdle`` governor currently used by the kernel can be read from the
:file:`current_governor_ro` (or :file:`current_governor` if
``cpuidle_sysfs_switch`` is present in the kernel command line) file under
:file:`/sys/devices/system/cpu/cpuidle/` in ``sysfs``.
......@@ -256,6 +256,8 @@ the ``menu`` governor by default and if it is not tickless, the default
``CPUIdle`` governor on it will be ``ladder``.
.. _menu-gov:
The ``menu`` Governor
=====================
......@@ -333,6 +335,92 @@ that time, the governor may need to select a shallower state with a suitable
target residency.
.. _teo-gov:
The Timer Events Oriented (TEO) Governor
========================================
The timer events oriented (TEO) governor is an alternative ``CPUIdle`` governor
for tickless systems. It follows the same basic strategy as the ``menu`` `one
<menu-gov_>`_: it always tries to find the deepest idle state suitable for the
given conditions. However, it applies a different approach to that problem.
First, it does not use sleep length correction factors, but instead it attempts
to correlate the observed idle duration values with the available idle states
and use that information to pick up the idle state that is most likely to
"match" the upcoming CPU idle interval. Second, it does not take the tasks
that were running on the given CPU in the past and are waiting on some I/O
operations to complete now at all (there is no guarantee that they will run on
the same CPU when they become runnable again) and the pattern detection code in
it avoids taking timer wakeups into account. It also only uses idle duration
values less than the current time till the closest timer (with the scheduler
tick excluded) for that purpose.
Like in the ``menu`` governor `case <menu-gov_>`_, the first step is to obtain
the *sleep length*, which is the time until the closest timer event with the
assumption that the scheduler tick will be stopped (that also is the upper bound
on the time until the next CPU wakeup). That value is then used to preselect an
idle state on the basis of three metrics maintained for each idle state provided
by the ``CPUIdle`` driver: ``hits``, ``misses`` and ``early_hits``.
The ``hits`` and ``misses`` metrics measure the likelihood that a given idle
state will "match" the observed (post-wakeup) idle duration if it "matches" the
sleep length. They both are subject to decay (after a CPU wakeup) every time
the target residency of the idle state corresponding to them is less than or
equal to the sleep length and the target residency of the next idle state is
greater than the sleep length (that is, when the idle state corresponding to
them "matches" the sleep length). The ``hits`` metric is increased if the
former condition is satisfied and the target residency of the given idle state
is less than or equal to the observed idle duration and the target residency of
the next idle state is greater than the observed idle duration at the same time
(that is, it is increased when the given idle state "matches" both the sleep
length and the observed idle duration). In turn, the ``misses`` metric is
increased when the given idle state "matches" the sleep length only and the
observed idle duration is too short for its target residency.
The ``early_hits`` metric measures the likelihood that a given idle state will
"match" the observed (post-wakeup) idle duration if it does not "match" the
sleep length. It is subject to decay on every CPU wakeup and it is increased
when the idle state corresponding to it "matches" the observed (post-wakeup)
idle duration and the target residency of the next idle state is less than or
equal to the sleep length (i.e. the idle state "matching" the sleep length is
deeper than the given one).
The governor walks the list of idle states provided by the ``CPUIdle`` driver
and finds the last (deepest) one with the target residency less than or equal
to the sleep length. Then, the ``hits`` and ``misses`` metrics of that idle
state are compared with each other and it is preselected if the ``hits`` one is
greater (which means that that idle state is likely to "match" the observed idle
duration after CPU wakeup). If the ``misses`` one is greater, the governor
preselects the shallower idle state with the maximum ``early_hits`` metric
(or if there are multiple shallower idle states with equal ``early_hits``
metric which also is the maximum, the shallowest of them will be preselected).
[If there is a wakeup latency constraint coming from the `PM QoS framework
<cpu-pm-qos_>`_ which is hit before reaching the deepest idle state with the
target residency within the sleep length, the deepest idle state with the exit
latency within the constraint is preselected without consulting the ``hits``,
``misses`` and ``early_hits`` metrics.]
Next, the governor takes several idle duration values observed most recently
into consideration and if at least a half of them are greater than or equal to
the target residency of the preselected idle state, that idle state becomes the
final candidate to ask for. Otherwise, the average of the most recent idle
duration values below the target residency of the preselected idle state is
computed and the governor walks the idle states shallower than the preselected
one and finds the deepest of them with the target residency within that average.
That idle state is then taken as the final candidate to ask for.
Still, at this point the governor may need to refine the idle state selection if
it has not decided to `stop the scheduler tick <idle-cpus-and-tick_>`_. That
generally happens if the target residency of the idle state selected so far is
less than the tick period and the tick has not been stopped already (in a
previous iteration of the idle loop). Then, like in the ``menu`` governor
`case <menu-gov_>`_, the sleep length used in the previous computations may not
reflect the real time until the closest timer event and if it really is greater
than that time, a shallower state with a suitable target residency may need to
be selected.
.. _idle-states-representation:
Representation of Idle States
......
......@@ -4,7 +4,7 @@ config CPU_IDLE
bool "CPU idle PM support"
default y if ACPI || PPC_PSERIES
select CPU_IDLE_GOV_LADDER if (!NO_HZ && !NO_HZ_IDLE)
select CPU_IDLE_GOV_MENU if (NO_HZ || NO_HZ_IDLE)
select CPU_IDLE_GOV_MENU if (NO_HZ || NO_HZ_IDLE) && !CPU_IDLE_GOV_TEO
help
CPU idle is a generic framework for supporting software-controlled
idle processor power management. It includes modular cross-platform
......@@ -23,6 +23,15 @@ config CPU_IDLE_GOV_LADDER
config CPU_IDLE_GOV_MENU
bool "Menu governor (for tickless system)"
config CPU_IDLE_GOV_TEO
bool "Timer events oriented (TEO) governor (for tickless systems)"
help
This governor implements a simplified idle state selection method
focused on timer events and does not do any interactivity boosting.
Some workloads benefit from using it and it generally should be safe
to use. Say Y here if you are not happy with the alternatives.
config DT_IDLE_STATES
bool
......
......@@ -4,3 +4,4 @@
obj-$(CONFIG_CPU_IDLE_GOV_LADDER) += ladder.o
obj-$(CONFIG_CPU_IDLE_GOV_MENU) += menu.o
obj-$(CONFIG_CPU_IDLE_GOV_TEO) += teo.o
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