Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 11 Jan 2022 08:54:18 +0100 | Subject | Re: [PATCH v2 1/3] sched/pelt: Don't sync hardly util_sum with uti_avg |
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On Fri, 7 Jan 2022 at 16:21, Vincent Guittot <vincent.guittot@linaro.org> wrote: > > On Fri, 7 Jan 2022 at 12:43, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: > > > > On 05/01/2022 14:57, Vincent Guittot wrote: > > > On Wed, 5 Jan 2022 at 14:15, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: > > >> > > >> On 04/01/2022 14:42, Vincent Guittot wrote: > > >>> On Tue, 4 Jan 2022 at 12:47, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: > > >>>> > > >>>> On 22/12/2021 10:38, Vincent Guittot wrote: > > >> > > >> [...] > > >> > > >>>> I still wonder whether the regression only comes from the changes in > > >>>> update_cfs_rq_load_avg(), introduced by 1c35b07e6d39. > > >>>> And could be fixed only by this part of the patch-set (A): > > >>> > > >>> I have been able to trigger the warning even with (A) though It took > > >>> much more time. > > >>> And I have been able to catch wrong situations (with additional > > >>> traces) in the 3 places A, B and C > > >> > > >> OK. By wrong situation you mean '_sum < _avg * MIN_DIVIDER' ? > > > > > > not only. > > > also util_sum == 0 but util_avg !=0 in different places although these > > > > Ah OK, I saw this one as part of '_sum < _avg * MIN_DIVIDER'. > > > > > situation didn't trigger sched_warn because some other sync happened > > > before the periodic call of __update_blocked_fair > > > or if nr_running == 1 and and task's util_avg/sum > cfs' util_avg/sum > > > just before removing the task > > > > I see. > > > > >>>> @@ -3677,15 +3706,22 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq > > >>>> *cfs_rq) > > >>>> > > >>>> r = removed_load; > > >>>> sub_positive(&sa->load_avg, r); > > >>>> - sa->load_sum = sa->load_avg * divider; > > >>>> + sub_positive(&sa->load_sum, r * divider); > > >>>> + sa->load_sum = max_t(u32, sa->load_sum, sa->load_avg * MIN_DIVIDER); > > >>>> > > >>>> r = removed_util; > > >>>> sub_positive(&sa->util_avg, r); > > >>>> - sa->util_sum = sa->util_avg * divider; > > >>>> + sub_positive(&sa->util_sum, r * divider); > > >>>> + sa->util_sum = max_t(u32, sa->util_sum, sa->util_avg * MIN_DIVIDER); > > >>>> > > >>>> r = removed_runnable; > > >>>> sub_positive(&sa->runnable_avg, r); > > >>>> - sa->runnable_sum = sa->runnable_avg * divider; > > >>>> + sub_positive(&sa->runnable_sum, r * divider); > > >>>> + sa->runnable_sum = max_t(u32, sa->runnable_sum, > > >>>> + sa->runnable_avg * MIN_DIVIDER); > > >>>> > > >>>> i.e. w/o changing update_tg_cfs_X() (and > > >>>> detach_entity_load_avg()/dequeue_load_avg()). > > >>>> > > >>>> update_load_avg() > > >>>> update_cfs_rq_load_avg() <--- > > >>>> propagate_entity_load_avg() > > >>>> update_tg_cfs_X() <--- > > >>>> > > >>>> > > >>>> I didn't see the SCHED_WARN_ON() [cfs_rq_is_decayed()] when looping on > > >>>> hackbench in several different sched group levels on > > >>>> [Hikey620 (Arm64, 8 CPUs, SMP, 4 taskgroups: A/B C/D E/F G/H), >12h uptime]. > > >>> > > >>> IIRC, it was with hikey960 with cgroup v1 > > >>> As a side note, I never trigger the problem with dragonboard845 and cgroup v2 > > >> > > >> OK, just started a test on hikey960 cgroupv1. Let's see if I can catch it. > > > > Still no sign of the issue (hikey960, cgroupv1, 4 taskgroups: A/B C/D > > E/F G/H > 45h uptime > > > > >>>>> @@ -3780,7 +3799,11 @@ static void detach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *s > > >>>>> > > >>>>> dequeue_load_avg(cfs_rq, se); > > >>>>> sub_positive(&cfs_rq->avg.util_avg, se->avg.util_avg); > > >>>>> - cfs_rq->avg.util_sum = cfs_rq->avg.util_avg * divider; > > >>>>> + sub_positive(&cfs_rq->avg.util_sum, se->avg.util_sum); > > >>>>> + /* See update_tg_cfs_util() */ > > >>>>> + cfs_rq->avg.util_sum = max_t(u32, cfs_rq->avg.util_sum, > > >>>>> + cfs_rq->avg.util_avg * MIN_DIVIDER); > > >>>>> + > > >>>> > > >>>> Maybe add a: > > >>>> > > >>>> Fixes: fcf6631f3736 ("sched/pelt: Ensure that *_sum is always synced > > >>>> with *_avg") > > >>> > > >>> I spent time thinking about adding fixes tag. There is no crash/warn > > >>> so far so should we propagate it back in LTS for better performance ? > > >> > > >> Not sure I understand. What do you mean by 'should we propagate it back > > >> in LTS'? > > > > > > Sorry I had any stables in mind and not only LTS. > > > > > > Some of the changes done in PELT signal propagation that replace > > > subtracting util_sum by using util_avg * divider instead, are related > > > to other problems with sched group hierarchy and > > > throttling/unthrottling. I'm not 100% confident that using fixes tag > > > to backport this on stables doesn't need to backport more patches on > > > other areas in order to not resurrect old problems. So I wonder if > > > it's worth backporting this on stables > > > > OK, I see. So only 1c35b07e6d39 (i.e. the util _sum/_avg change in > > update_cfs_rq_load_avg() (1)) caused the CPU frequency regression. That > > was the reason why I initially suggested to split the patch-set > > differently. But you said that you saw the issue also when (1) is fixed. > > Ok, I think that we were not speaking about the same setup. I wrongly > read that you were saying that > sa->util_sum = max_t(u32, sa->util_sum, sa->util_avg * MIN_DIVIDER); > was only needed in update_cfs_rq_load_avg() but not in the other places. > > But what you said is that we only need the below to fix the perf > regression raised by rick ? > r = removed_util; > sub_positive(&sa->util_avg, r); > - sa->util_sum = sa->util_avg * divider; > + sub_positive(&sa->util_sum, r * divider); > + sa->util_sum = max_t(u32, sa->util_sum, sa->util_avg * MIN_DIVIDER);
The test with the code above doesn't trigger any SCHEd_WARN over the weekend so it's probably ok to make a dedicated patch for this with tag. I'm going to prepare a v2
> > The WARN that I mentioned in my previous email was about not adding > the max_t in all 3 places. I rerun some test today and I triggered the > WARN after a detach without the max_t line. > > I can probably isolate the code above in a dedicated patch for the > regression raised by Rick and we could consider adding a fixes tag; I > will run more tests with only this part during the weekend. > That being said, we need to stay consistent in all 3 places where we > move or propagate some *_avg. In particular, using "sa->util_sum = > sa->util_avg * divider" has the drawback of filtering the contribution > not already accounted for in util_avg and the impact is much larger > than expected. It means that although fixing only > update_cfs_rq_load_avg() seems enough for rick's case, some other > cases could be impacted by the 2 other places and we need to fixed > them now that we have a better view of the root cause
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