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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Fri, 24 Apr 2020 10:54:08 +0200
Message-ID: <CAKfTPtB4Uf7PbRdrqLm8vMy-UfYhQV_SgFSKCP2QOyvxSqLtqg@mail.gmail.com>
Subject: Re: [PATCH] sched/pelt: sync util/runnable_sum with PELT window when propagating
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.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 Fri, 24 Apr 2020 at 10:41, Dietmar Eggemann <dietmar.eggemann@arm.com> wrote:
>
> On 24/04/2020 09:37, Vincent Guittot wrote:
> > 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
>
> But IMHO the
>
> "* however, the PELT windows are not aligned between grq and gse."
>
> should only apply to update_tg_cfs_util() and update_tg_cfs_runnable().
> And attach_entity_load_avg() (for cfs_rq and se).
>
> They seem to be special since we derive divider from a cfs_rq PELT value
> and use it for a se PELT value.

hmmm... There is nothing special here.

When se is attached to cfs_rq, they both have the same divider because
they use the same clock.

>
> I assume this fact is specifically worth highlighting with a comment. I
> mean the fact we can do this because the decay windows are actually aligned.