From: Vincent Guittot Date: Wed, 26 Apr 2017 06:27:56 +0000 (+0200) Subject: sched/cfs: Make util/load_avg more stable X-Git-Url: http://git.lede-project.org./?a=commitdiff_plain;h=625ed2bf049d5a352c1bcca962d6e133454eaaff;p=openwrt%2Fstaging%2Fblogic.git 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 Signed-off-by: Vincent Guittot Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Mike Galbraith Cc: Morten.Rasmussen@arm.com Cc: Thomas Gleixner 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 --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d71109321841..4f1825d60937 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; }