Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 25 Apr 2017 17:17:08 +0200 | Subject | Re: [PATCH v2] sched/cfs: make util/load_avg more stable |
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On 25 April 2017 at 16:53, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: > On 25/04/17 13:40, Vincent Guittot wrote: >> On 25 April 2017 at 13:05, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: >>> 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. >> >> This refers to the very beginning and very end of time segment formulas below. >> That being said, 1024 is not reachable because at very end we have : >> 1024*MAX_LOAD_AVG*y+1024*1023 = 1023,9997 >> >> 1002 is not reachable because at very beg we have >> 1024*MAX_LOAD_AVG*y+ 1024*0 = 1002,0577 >> >> But we are working with integer so [1002..1024[ is probably more correct > > OK, this is with y = 32nd-rt(0.5) exactly, understood. > > I assume you mean LOAD_AVG_MAX instead of MAX_LOAD_AVG.
correct
> >>> 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. > > s/MAX_LOAD_AVG/LOAD_AVG_MAX
ditto
> >>>> >>>> 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? >> >> removing its contribution refers to "- sa->period_contrib * weight" >> and "- (running * sa->period_contrib << SCHED_CAPACITY_SHIFT))" in >> patch 1/2 of the previous version > > Yup, makes sense, so the '-1024' is the influence of the current 'time > segment' (n = 0) then.
Yes
> > IMHO, the removing of contribution in patch 1/2 wouldn't take freq and > cpu scaling of contribution (which is still in accumulate_sum()) into > consideration.
Yes you're right but as everything has been put in 1 single patch in v2, it doesn't make any difference now
> >>>> 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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