Messages in this thread Patch in this message | | | From | Vincent Guittot <> | Date | Wed, 5 Jan 2022 14:57:45 +0100 | Subject | Re: [PATCH v2 1/3] sched/pelt: Don't sync hardly util_sum with uti_avg |
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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 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
> > >> @@ -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. > > [...] > > >>> @@ -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
> > [...] > > >> This max_t() should make sure that `_sum is always >= _avg * > >> MIN_DIVIDER`. Which is not the case sometimes. Currently this is done in > >> > >> (1) update_cfs_rq_load_avg() > >> (2) detach_entity_load_avg() and dequeue_load_avg() > >> (3) update_tg_cfs_X() > >> > >> but not in attach_entity_load_avg(), enqueue_load_avg(). What's the > >> reason for this? > > > > Main reason is that I have never seen the problem. > > Then, the problem comes from subtracting task's value whereas here we > > always add positive value > > OK, I see. The add_positive()'s in update_tg_cfs_X() deal w/ `long` values.
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