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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 24 Apr 2020 09:37:31 +0200
Message-ID: <CAKfTPtBUtK1c2WvE82k1bpS6hWECBs25rRc6t_6gyeJWRaF8uA@mail.gmail.com>
Subject: Re: [PATCH] sched/pelt: sync util/runnable_sum with PELT window when propagating
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Thu, 23 Apr 2020 at 21:29, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Wed, Apr 22, 2020 at 05:14:01PM +0200, Vincent Guittot wrote:
> > update_tg_cfs_util/runnable() propagate the impact of the attach/detach of
> > an entity down into the cfs_rq hierarchy which must keep the sync with
> > the current pelt window.
> >
> > Even if we can't sync child rq and its group se, we can sync the group se
> > and parent cfs_rq with current PELT window. In fact, we must keep them sync
> > in order to stay also synced with others se and group se that are already
> > attached to the cfs_rq.
> >
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> > ---
> >  kernel/sched/fair.c | 26 ++++++--------------------
> >  1 file changed, 6 insertions(+), 20 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 02f323b85b6d..ca6aa89c88f2 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -3441,52 +3441,38 @@ 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;
> > +     u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib;
> >
> >       /* Nothing to update */
> >       if (!delta)
> >               return;
> >
> > -     /*
> > -      * The relation between sum and avg is:
> > -      *
> > -      *   LOAD_AVG_MAX - 1024 + sa->period_contrib
> > -      *
> > -      * however, the PELT windows are not aligned between grq and gse.
> > -      */
>
> Instead of deleting this, could we perhaps extend it?

In fact, this is not the only place in fair.c that uses this rule to
align _avg and _sum but others don't have any special comment.

I can add a more detailed description of this relation for
___update_load_avg() in pelt.c and make a ref to this in all places in
fair.c that use this rule which are :
- update_tg_cfs_util
- update_tg_cfs_runnable
- update_cfs_rq_load_avg
- attach_entity_load_avg
- reweight_entity