kernel/git/stable/linux.git/Documentation/scheduler, branch linux-4.19.y

sched/fair: Fix low cpu usage with high throttling by removing expiration of cpu-local slices

2019-11-12T18:20:50+00:00

commit de53fd7aedb100f03e5d2231cfce0e4993282425 upstream. It has been observed, that highly-threaded, non-cpu-bound applications running under cpu.cfs_quota_us constraints can hit a high percentage of periods throttled while simultaneously not consuming the allocated amount of quota. This use case is typical of user-interactive non-cpu bound applications, such as those running in kubernetes or mesos when run on multiple cpu cores. This has been root caused to cpu-local run queue being allocated per cpu bandwidth slices, and then not fully using that slice within the period. At which point the slice and quota expires. This expiration of unused slice results in applications not being able to utilize the quota for which they are allocated. The non-expiration of per-cpu slices was recently fixed by 'commit 512ac999d275 ("sched/fair: Fix bandwidth timer clock drift condition")'. Prior to that it appears that this had been broken since at least 'commit 51f2176d74ac ("sched/fair: Fix unlocked reads of some cfs_b->quota/period")' which was introduced in v3.16-rc1 in 2014. That added the following conditional which resulted in slices never being expired. if (cfs_rq->runtime_expires != cfs_b->runtime_expires) { /* extend local deadline, drift is bounded above by 2 ticks */ cfs_rq->runtime_expires += TICK_NSEC; Because this was broken for nearly 5 years, and has recently been fixed and is now being noticed by many users running kubernetes (https://github.com/kubernetes/kubernetes/issues/67577) it is my opinion that the mechanisms around expiring runtime should be removed altogether. This allows quota already allocated to per-cpu run-queues to live longer than the period boundary. This allows threads on runqueues that do not use much CPU to continue to use their remaining slice over a longer period of time than cpu.cfs_period_us. However, this helps prevent the above condition of hitting throttling while also not fully utilizing your cpu quota. This theoretically allows a machine to use slightly more than its allotted quota in some periods. This overflow would be bounded by the remaining quota left on each per-cpu runqueueu. This is typically no more than min_cfs_rq_runtime=1ms per cpu. For CPU bound tasks this will change nothing, as they should theoretically fully utilize all of their quota in each period. For user-interactive tasks as described above this provides a much better user/application experience as their cpu utilization will more closely match the amount they requested when they hit throttling. This means that cpu limits no longer strictly apply per period for non-cpu bound applications, but that they are still accurate over longer timeframes. This greatly improves performance of high-thread-count, non-cpu bound applications with low cfs_quota_us allocation on high-core-count machines. In the case of an artificial testcase (10ms/100ms of quota on 80 CPU machine), this commit resulted in almost 30x performance improvement, while still maintaining correct cpu quota restrictions. That testcase is available at https://github.com/indeedeng/fibtest. Fixes: 512ac999d275 ("sched/fair: Fix bandwidth timer clock drift condition") Signed-off-by: Dave Chiluk Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Phil Auld Reviewed-by: Ben Segall Cc: Ingo Molnar Cc: John Hammond Cc: Jonathan Corbet Cc: Kyle Anderson Cc: Gabriel Munos Cc: Peter Oskolkov Cc: Cong Wang Cc: Brendan Gregg Link: https://lkml.kernel.org/r/1563900266-19734-2-git-send-email-chiluk+linux@indeed.com Signed-off-by: Greg Kroah-Hartman

sched/fair: Fix "runnable_avg_yN_inv" not used warnings

2019-07-26T07:14:08+00:00

[ Upstream commit 509466b7d480bc5d22e90b9fbe6122ae0e2fbe39 ] runnable_avg_yN_inv[] is only used in kernel/sched/pelt.c but was included in several other places because they need other macros all came from kernel/sched/sched-pelt.h which was generated by Documentation/scheduler/sched-pelt. As the result, it causes compilation a lot of warnings, kernel/sched/sched-pelt.h:4:18: warning: 'runnable_avg_yN_inv' defined but not used [-Wunused-const-variable=] kernel/sched/sched-pelt.h:4:18: warning: 'runnable_avg_yN_inv' defined but not used [-Wunused-const-variable=] kernel/sched/sched-pelt.h:4:18: warning: 'runnable_avg_yN_inv' defined but not used [-Wunused-const-variable=] ... Silence it by appending the __maybe_unused attribute for it, so all generated variables and macros can still be kept in the same file. Signed-off-by: Qian Cai Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: https://lkml.kernel.org/r/1559596304-31581-1-git-send-email-cai@lca.pw Signed-off-by: Ingo Molnar Signed-off-by: Sasha Levin

sched/deadline/Documentation: Add overrun signal and GRUB-PA documentation

2018-05-14T07:12:27+00:00

Signed-off-by: Claudio Scordino Signed-off-by: Peter Zijlstra (Intel) Cc: Jonathan Corbet Cc: Juri Lelli Cc: Linus Torvalds Cc: Luca Abeni Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-kernel@vger.kernel.org Link: http://lkml.kernel.org/r/1522741362-4542-1-git-send-email-claudio@evidence.eu.com Signed-off-by: Ingo Molnar

sched/deadline: Fix the description of runtime accounting in the documentation

2017-11-16T08:00:35+00:00

Signed-off-by: Claudio Scordino Signed-off-by: Luca Abeni Acked-by: Daniel Bristot de Oliveira Acked-by: Peter Zijlstra Cc: Jonathan Corbet Cc: Linus Torvalds Cc: Mathieu Poirier Cc: Thomas Gleixner Cc: Tommaso Cucinotta Cc: linux-doc@vger.kernel.org Link: http://lkml.kernel.org/r/1510658366-28995-1-git-send-email-claudio@evidence.eu.com Signed-off-by: Ingo Molnar

sched/deadline: Add documentation about GRUB reclaiming

2017-06-08T08:31:56+00:00

This patch adds the documentation about the GRUB reclaiming algorithm, adding a few details discussed in list. Signed-off-by: Claudio Scordino Signed-off-by: Luca Abeni Signed-off-by: Peter Zijlstra (Intel) Cc: Daniel Bristot de Oliveira Cc: Joel Fernandes Cc: Juri Lelli Cc: Linus Torvalds Cc: Mathieu Poirier Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Thomas Gleixner Cc: Tommaso Cucinotta Link: http://lkml.kernel.org/r/1495138417-6203-11-git-send-email-luca.abeni@santannapisa.it Signed-off-by: Ingo Molnar

sched/Documentation: Add 'sched-pelt' tool

2017-04-14T08:26:35+00:00

Add a user-space program to compute/generate the PELT constants. The kernel/sched/sched-pelt.h header will contain the output of this program. Signed-off-by: Yuyang Du Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: bsegall@google.com Cc: dietmar.eggemann@arm.com Cc: matt@codeblueprint.co.uk Cc: morten.rasmussen@arm.com Cc: pjt@google.com Cc: umgwanakikbuti@gmail.com Cc: vincent.guittot@linaro.org Link: http://lkml.kernel.org/r/1486935863-25251-2-git-send-email-yuyang.du@intel.com Signed-off-by: Ingo Molnar

sched/Documentation/sched-rt-group: Fix incorrect example

2017-01-22T09:34:17+00:00

I feel that the example given in the document to show the possibility of task starvation of configurable period is wrong. The example says group A and B both have 50% bandwidth, and a while (1) loop in A will run for the full period of B and can starve B's tasks. So I think the runtime of group A should be 50000us, then the period and runtime of group B should be 50000us and 25000us. Signed-off-by: Zhou Chengming Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: corbet@lwn.net Cc: iamyooon@gmail.com Cc: linux-doc@vger.kernel.org Cc: lizefan@huawei.com Link: http://lkml.kernel.org/r/1485069755-44287-1-git-send-email-zhouchengming1@huawei.com Signed-off-by: Ingo Molnar

sched/deadline: Show leftover runtime and abs deadline in /proc/*/sched

2017-01-14T10:30:00+00:00

This patch allows for reading the current (leftover) runtime and absolute deadline of a SCHED_DEADLINE task through /proc/*/sched (entries dl.runtime and dl.deadline), while debugging/testing. Signed-off-by: Tommaso Cucinotta Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Juri Lelli Reviewed-by: Luca Abeni Acked-by: Daniel Bistrot de Oliveira Cc: Juri Lelli Cc: Linus Torvalds Cc: Mike Galbraith Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/1477473437-10346-2-git-send-email-tommaso.cucinotta@sssup.it Signed-off-by: Ingo Molnar

docs/completion.txt: drop dangling reference to completions-design.txt

2016-11-16T23:27:50+00:00

Per the original author, the proposed document was never deemed necessary, and the important bits got merged into completion.txt. Let's just stop confusing readers by pointing at a nonexistent doc. Signed-off-by: Brian Norris Signed-off-by: Jonathan Corbet

sched/deadline: Document behavior of sched_yield()

2016-09-10T09:17:41+00:00

This is a documentation only patch, explaining the behavior of sched_yield() when a SCHED_DEADLINE task calls it (give up remaining runtime and be throttled until next period begins). Signed-off-by: Tommaso Cucinotta Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Juri Lelli Reviewed-by: Luca Abeni Reviewed-by: Daniel Bristot de Oliveira Cc: Juri Lelli Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: linux-dl@retis.sssup.it Link: http://lkml.kernel.org/r/1473443117-11794-2-git-send-email-tommaso.cucinotta@sssup.it Signed-off-by: Ingo Molnar