Commit d73df887 authored by Huaixin Chang's avatar Huaixin Chang Committed by Peter Zijlstra

sched/fair: Add document for burstable CFS bandwidth

Basic description of usage and effect for CFS Bandwidth Control Burst.
Co-developed-by: default avatarShanpei Chen <shanpeic@linux.alibaba.com>
Signed-off-by: default avatarShanpei Chen <shanpeic@linux.alibaba.com>
Co-developed-by: default avatarTianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: default avatarTianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: default avatarHuaixin Chang <changhuaixin@linux.alibaba.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarDaniel Jordan <daniel.m.jordan@oracle.com>
Acked-by: default avatarTejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20210830032215.16302-3-changhuaixin@linux.alibaba.com
parent bcb1704a
......@@ -1016,6 +1016,8 @@ All time durations are in microseconds.
- nr_periods
- nr_throttled
- throttled_usec
- nr_bursts
- burst_usec
cpu.weight
A read-write single value file which exists on non-root
......@@ -1047,6 +1049,12 @@ All time durations are in microseconds.
$PERIOD duration. "max" for $MAX indicates no limit. If only
one number is written, $MAX is updated.
cpu.max.burst
A read-write single value file which exists on non-root
cgroups. The default is "0".
The burst in the range [0, $MAX].
cpu.pressure
A read-write nested-keyed file.
......
......@@ -22,9 +22,52 @@ cfs_quota units at each period boundary. As threads consume this bandwidth it
is transferred to cpu-local "silos" on a demand basis. The amount transferred
within each of these updates is tunable and described as the "slice".
Burst feature
-------------
This feature borrows time now against our future underrun, at the cost of
increased interference against the other system users. All nicely bounded.
Traditional (UP-EDF) bandwidth control is something like:
(U = \Sum u_i) <= 1
This guaranteeds both that every deadline is met and that the system is
stable. After all, if U were > 1, then for every second of walltime,
we'd have to run more than a second of program time, and obviously miss
our deadline, but the next deadline will be further out still, there is
never time to catch up, unbounded fail.
The burst feature observes that a workload doesn't always executes the full
quota; this enables one to describe u_i as a statistical distribution.
For example, have u_i = {x,e}_i, where x is the p(95) and x+e p(100)
(the traditional WCET). This effectively allows u to be smaller,
increasing the efficiency (we can pack more tasks in the system), but at
the cost of missing deadlines when all the odds line up. However, it
does maintain stability, since every overrun must be paired with an
underrun as long as our x is above the average.
That is, suppose we have 2 tasks, both specify a p(95) value, then we
have a p(95)*p(95) = 90.25% chance both tasks are within their quota and
everything is good. At the same time we have a p(5)p(5) = 0.25% chance
both tasks will exceed their quota at the same time (guaranteed deadline
fail). Somewhere in between there's a threshold where one exceeds and
the other doesn't underrun enough to compensate; this depends on the
specific CDFs.
At the same time, we can say that the worst case deadline miss, will be
\Sum e_i; that is, there is a bounded tardiness (under the assumption
that x+e is indeed WCET).
The interferenece when using burst is valued by the possibilities for
missing the deadline and the average WCET. Test results showed that when
there many cgroups or CPU is under utilized, the interference is
limited. More details are shown in:
https://lore.kernel.org/lkml/5371BD36-55AE-4F71-B9D7-B86DC32E3D2B@linux.alibaba.com/
Management
----------
Quota and period are managed within the cpu subsystem via cgroupfs.
Quota, period and burst are managed within the cpu subsystem via cgroupfs.
.. note::
The cgroupfs files described in this section are only applicable
......@@ -32,29 +75,37 @@ Quota and period are managed within the cpu subsystem via cgroupfs.
:ref:`Documentation/admin-guide/cgroup-v2.rst <cgroup-v2-cpu>`.
- cpu.cfs_quota_us: the total available run-time within a period (in
microseconds)
- cpu.cfs_quota_us: run-time replenished within a period (in microseconds)
- cpu.cfs_period_us: the length of a period (in microseconds)
- cpu.stat: exports throttling statistics [explained further below]
- cpu.cfs_burst_us: the maximum accumulated run-time (in microseconds)
The default values are::
cpu.cfs_period_us=100ms
cpu.cfs_quota=-1
cpu.cfs_quota_us=-1
cpu.cfs_burst_us=0
A value of -1 for cpu.cfs_quota_us indicates that the group does not have any
bandwidth restriction in place, such a group is described as an unconstrained
bandwidth group. This represents the traditional work-conserving behavior for
CFS.
Writing any (valid) positive value(s) will enact the specified bandwidth limit.
The minimum quota allowed for the quota or period is 1ms. There is also an
upper bound on the period length of 1s. Additional restrictions exist when
bandwidth limits are used in a hierarchical fashion, these are explained in
more detail below.
Writing any (valid) positive value(s) no smaller than cpu.cfs_burst_us will
enact the specified bandwidth limit. The minimum quota allowed for the quota or
period is 1ms. There is also an upper bound on the period length of 1s.
Additional restrictions exist when bandwidth limits are used in a hierarchical
fashion, these are explained in more detail below.
Writing any negative value to cpu.cfs_quota_us will remove the bandwidth limit
and return the group to an unconstrained state once more.
A value of 0 for cpu.cfs_burst_us indicates that the group can not accumulate
any unused bandwidth. It makes the traditional bandwidth control behavior for
CFS unchanged. Writing any (valid) positive value(s) no larger than
cpu.cfs_quota_us into cpu.cfs_burst_us will enact the cap on unused bandwidth
accumulation.
Any updates to a group's bandwidth specification will result in it becoming
unthrottled if it is in a constrained state.
......@@ -74,7 +125,7 @@ for more fine-grained consumption.
Statistics
----------
A group's bandwidth statistics are exported via 3 fields in cpu.stat.
A group's bandwidth statistics are exported via 5 fields in cpu.stat.
cpu.stat:
......@@ -82,6 +133,9 @@ cpu.stat:
- nr_throttled: Number of times the group has been throttled/limited.
- throttled_time: The total time duration (in nanoseconds) for which entities
of the group have been throttled.
- nr_bursts: Number of periods burst occurs.
- burst_time: Cumulative wall-time (in nanoseconds) that any CPUs has used
above quota in respective periods
This interface is read-only.
......@@ -179,3 +233,15 @@ Examples
By using a small period here we are ensuring a consistent latency
response at the expense of burst capacity.
4. Limit a group to 40% of 1 CPU, and allow accumulate up to 20% of 1 CPU
additionally, in case accumulation has been done.
With 50ms period, 20ms quota will be equivalent to 40% of 1 CPU.
And 10ms burst will be equivalent to 20% of 1 CPU.
# echo 20000 > cpu.cfs_quota_us /* quota = 20ms */
# echo 50000 > cpu.cfs_period_us /* period = 50ms */
# echo 10000 > cpu.cfs_burst_us /* burst = 10ms */
Larger buffer setting (no larger than quota) allows greater burst capacity.
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