Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 12 Jan 2022 17:04:56 +0100 | Subject | Re: [PATCH 3 1/4] sched/pelt: Relax the sync of util_sum with util_avg |
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On Wed, 12 Jan 2022 at 16:26, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote: > > On 11/01/2022 14:46, Vincent Guittot wrote: > > Rick reported performance regressions in bugzilla because of cpu frequency > > being lower than before: > > https://bugzilla.kernel.org/show_bug.cgi?id=215045 > > > > He bisected the problem to: > > commit 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent") > > > > This commit forces util_sum to be synced with the new util_avg after > > removing the contribution of a task and before the next periodic sync. By > > doing so util_sum is rounded to its lower bound and might lost up to > > LOAD_AVG_MAX-1 of accumulated contribution which has not yet been > > reflected in util_avg. > > > > Instead of always setting util_sum to the low bound of util_avg, which can > > significantly lower the utilization of root cfs_rq after propagating the > > change down into the hierarchy, we revert the change of util_sum and > > propagate the difference. > > IMHO, this paragraph does not match the changes in this patch.
> > > In addition, we also check that cfs's util_sum always stays above the > > lower bound for a given util_avg as it has been observed that > > sched_entity's util_sum is sometimes above cfs one. > > And I guess this one also refers to the code change in 2/4, i.e. in > update_tg_cfs_util().
The 3 places can have the problem although it's probably more obvious in detach_entity_load_avg or update_tg_cfs_util because this is done synchronously unlike here
> > Reported-by: Rick Yiu <rickyiu@google.com> > > Fixes: 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent") > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > > --- > > kernel/sched/fair.c | 16 ++++++++++++++-- > > 1 file changed, 14 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 095b0aa378df..ed35255fdb85 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -3381,6 +3381,7 @@ void set_task_rq_fair(struct sched_entity *se, > > se->avg.last_update_time = n_last_update_time; > > } > > > > +#define MIN_DIVIDER (LOAD_AVG_MAX - 1024) > > > > /* > > * When on migration a sched_entity joins/leaves the PELT hierarchy, we need to > > @@ -3449,7 +3450,6 @@ void set_task_rq_fair(struct sched_entity *se, > > * XXX: only do this for the part of runnable > running ? > > * > > */ > > - > > static inline void > > update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq) > > { > > @@ -3681,7 +3681,19 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq) > > > > r = removed_util; > > sub_positive(&sa->util_avg, r); > > - sa->util_sum = sa->util_avg * divider; > > + sub_positive(&sa->util_sum, r * divider); > > + /* > > + * Because of rounding, se->util_sum might ends up being +1 more than > > There are no se's involved in update_cfs_rq_load_avg(). Could be hard to > grasp for people only looking at this function.
I moved the comment there to stay in patch 1 but it reflects the whole problem of rounding when removing some util_avg/sum from a cfs whereas we only have part of the problem in update_cfs_rq_load_avg()
> > > + * cfs->util_sum. Although this is not a problem by itself, detaching > > + * a lot of tasks with the rounding problem between 2 updates of > > + * util_avg (~1ms) can make cfs->util_sum becoming null whereas > > + * cfs_util_avg is not. > > + * Check that util_sum is still above its lower bound for the new > > + * util_avg. Given that period_contrib might have moved since the last > > + * sync, we are only sure that util_sum must be above or equal to > > + * util_avg * minimum possible divider > ^^^ > some superfluous whitepaces. > > > + */ > > + sa->util_sum = max_t(u32, sa->util_sum, sa->util_avg * MIN_DIVIDER); > > > > r = removed_runnable; > > sub_positive(&sa->runnable_avg, r); > > >
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