Messages in this thread | | | Subject | Re: [PATCH v2] sched/cfs: make util/load_avg more stable | From | Dietmar Eggemann <> | Date | Tue, 25 Apr 2017 12:05:16 +0100 |
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On 19/04/17 17:54, Vincent Guittot wrote: > 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
Why do you use the square brackets the other way around? Just curious.
1002 stands for 1024*y^1 w/ y = 4008/4096 or y^32 = 0.5, right ? Might be worth mentioning.
> 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. Bu 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 MAX_LOAD_AVG*y. At the very end of the current > time segment, the max value becomes 1024(us) + MAX_LOAD_AVG*y which is equal > to MAX_LOAD_AVG. In fact, the max value is > sa->period_contrib + MAX_LOAD_AVG*y at any time in the time segment. > > Taking advantage of the fact that MAX_LOAD_AVG*y == MAX_LOAD_AVG-1024, the > range becomes [0..MAX_LOAD_AVG-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.
Removing its contribution stands for '- 1024' of 'LOAD_AVG_MAX - 1024' which was added in patch 1/2?
> > Suggested-by: Peter Zijlstra <peterz@infradead.org> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > > Fold both patches in one > > 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 3f83a35..c3b8f0f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -3017,12 +3017,12 @@ ___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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