Messages in this thread Patch in this message | | | Date | Mon, 15 May 2017 01:55:44 -0700 | From | tip-bot for Vincent Guittot <> | Subject | [tip:sched/core] sched/cfs: Make util/load_avg more stable |
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Commit-ID: 625ed2bf049d5a352c1bcca962d6e133454eaaff Gitweb: http://git.kernel.org/tip/625ed2bf049d5a352c1bcca962d6e133454eaaff Author: Vincent Guittot <vincent.guittot@linaro.org> AuthorDate: Wed, 26 Apr 2017 08:27:56 +0200 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Mon, 15 May 2017 10:15:13 +0200
sched/cfs: Make util/load_avg more stable
In the current implementation of load/util_avg, we assume that the ongoing time segment has fully elapsed, and util/load_sum is divided by LOAD_AVG_MAX, even if part of the time segment still remains to run. As a consequence, this remaining part is considered as idle time and generates unexpected variations of util_avg of a busy CPU in the range [1002..1024[ whereas util_avg should stay at 1023.
In order to keep the metric stable, we should not consider the ongoing time segment when computing load/util_avg but only the segments that have already fully elapsed. But to not consider the current time segment adds unwanted latency in the load/util_avg responsivness especially when the time is scaled instead of the contribution.
Instead of waiting for the current time segment to have fully elapsed before accounting it in load/util_avg, we can already account the elapsed part but change the range used to compute load/util_avg accordingly.
At the very beginning of a new time segment, the past segments have been decayed and the max value is LOAD_AVG_MAX*y. At the very end of the current time segment, the max value becomes:
LOAD_AVG_MAX*y + 1024(us) (== LOAD_AVG_MAX)
In fact, the max value is:
LOAD_AVG_MAX*y + sa->period_contrib
at any time in the time segment.
Taking advantage of the fact that:
LOAD_AVG_MAX*y == LOAD_AVG_MAX-1024
the range becomes [0..LOAD_AVG_MAX-1024+sa->period_contrib].
As the elapsed part is already accounted in load/util_sum, we update the max value according to the current position in the time segment instead of removing its contribution.
Suggested-by: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mike Galbraith <efault@gmx.de> Cc: Morten.Rasmussen@arm.com Cc: Thomas Gleixner <tglx@linutronix.de> Cc: bsegall@google.com Cc: dietmar.eggemann@arm.com Cc: pjt@google.com Cc: yuyang.du@intel.com Link: http://lkml.kernel.org/r/1493188076-2767-1-git-send-email-vincent.guittot@linaro.org Signed-off-by: Ingo Molnar <mingo@kernel.org> --- kernel/sched/fair.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d711093..4f1825d 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -2916,12 +2916,12 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa, /* * Step 2: update *_avg. */ - sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX); + sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib); if (cfs_rq) { cfs_rq->runnable_load_avg = - div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX); + div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib); } - sa->util_avg = sa->util_sum / LOAD_AVG_MAX; + sa->util_avg = sa->util_sum / (LOAD_AVG_MAX - 1024 + sa->period_contrib); return 1; }
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