Re: [PATCH 3 1/4] sched/pelt: Relax the sync of util_sum with util_avg

[Vincent Guittot]

On Wed, 12 Jan 2022 at 16:26, Dietmar Eggemann <dietmar.eggemann@xxxxxxx> 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@xxxxxxxxxx>
> > Fixes: 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent")
> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > ---
> >  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);
> >
>