linux-kernel - Re: [PATCH v2] sched/pelt: sync util/runnable

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Date:   Tue, 19 May 2020 17:41:19 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched/pelt: sync util/runnable_sum with PELT window
 when propagating

On Tue, 19 May 2020 at 12:28, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> On 06/05/2020 17:53, Vincent Guittot wrote:
>
> [...]
>
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 02f323b85b6d..df3923a65162 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3441,52 +3441,46 @@ static inline void
> >  update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq)
> >  {
> >       long delta = gcfs_rq->avg.util_avg - se->avg.util_avg;
> > +     /*
> > +      * cfs_rq->avg.period_contrib can be used for both cfs_rq and se.
> > +      * See ___update_load_avg() for details.
> > +      */
> > +     u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib;
>
> Why not doing the assignment (like in update_tg_cfs_load()) after the
> next condition? Same question for update_tg_cfs_runnable().

In fact, I expect the compiler to be smart enough to do this at the best place

>
> [...]
>
> >  static inline void
> >  update_tg_cfs_runnable(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq)
> >  {
> >       long delta = gcfs_rq->avg.runnable_avg - se->avg.runnable_avg;
> > +     /*
> > +      * cfs_rq->avg.period_contrib can be used for both cfs_rq and se.
> > +      * See ___update_load_avg() for details.
> > +      */
> > +     u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib;
>
> We know have 6 assignments like this in fair.c and 1 in pelt.c. Could
> this not be refactored by using something like this in pelt.h:
>
> +static inline u32 get_divider(struct sched_avg *avg)

That's a good point
I would add a pelt in the name like
static inline u32 get_pelt_divider(struct sched_avg *avg)

> +{
> +       return LOAD_AVG_MAX - 1024 + avg->period_contrib;
> +}
>
> [...]
>
> > diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
> > index b647d04d9c8b..1feff80e7e45 100644
> > --- a/kernel/sched/pelt.c
> > +++ b/kernel/sched/pelt.c
> > @@ -237,6 +237,30 @@ ___update_load_sum(u64 now, struct sched_avg *sa,
> >       return 1;
> >  }
> >
> > +/*
> > + * When syncing *_avg with *_sum, we must take into account the current
> > + * position in the PELT segment otherwise the remaining part of the segment
> > + * will be considered as idle time whereas it's not yet elapsed and this will
> > + * generate unwanted oscillation in the range [1002..1024[.
> > + *
> > + * The max value of *_sum varies with the position in the time segment and is
> > + * equals to :
> > + *
> > + *   LOAD_AVG_MAX*y + sa->period_contrib
> > + *
> > + * which can be simplified into:
> > + *
> > + *   LOAD_AVG_MAX - 1024 + sa->period_contrib
> > + *
> > + * because LOAD_AVG_MAX*y == LOAD_AVG_MAX-1024
>
> Isn't this rather '~' instead of '==', even for y^32 = 0.5 ?
>
> 47742 * 0.5^(1/32) ~ 47742 - 1024

With integer precision and the runnable_avg_yN_inv array, you've got
exactly 1024

>
>
> Apart from that, LGTM
>
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>