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Message-ID: <5e21452a727dcd6d3257496a2c42f49bd16e9cb5.camel@surriel.com>
Subject: Re: [PATCH v2] sched,fair: skip newidle_balance if a wakeup is
 pending
From:   Rik van Riel <riel@surriel.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Kernel Team <kernel-team@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <valentin.schneider@arm.com>
Date:   Tue, 20 Apr 2021 11:20:36 -0400
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References: <20210419125134.5cab12ea@imladris.surriel.com>
         <CAKfTPtDjEMJeoZgE1an9Nh9QZPc2gJetsZHL+8QAWzqX5_znvw@mail.gmail.com>
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On Tue, 2021-04-20 at 11:04 +0200, Vincent Guittot wrote:
> On Mon, 19 Apr 2021 at 18:51, Rik van Riel <riel@surriel.com> wrote:
> >=20
> > @@ -10688,7 +10697,7 @@ static int newidle_balance(struct rq
> > *this_rq, struct rq_flags *rf)
> >         if (this_rq->nr_running !=3D this_rq->cfs.h_nr_running)
> >                 pulled_task =3D -1;
> >=20
> > -       if (pulled_task)
> > +       if (pulled_task || this_rq->ttwu_pending)
>=20
> This needs at least a comment to explain why we must clear
> this_rq->idle_stamp when this_rq->ttwu_pending is set whereas it is
> also done during sched_ttwu_pending()
>=20
> >                 this_rq->idle_stamp =3D 0;

I spent some time staring at sched_ttwu_pending and
the functions it calls, but I can't seem to spot
where it clears rq->idle_stamp, except inside
ttwu_do_wakeup where it will end up adding a
non-idle period into the rq->avg_idle, which seems
wrong.

If we are actually idle, and get woken up with a
ttwu_queue task, we do not come through newidle_balance,
and we end up counting the idle time into the avg_idle
number.

However, if a task is woken up while the CPU is
in newidle_balance, because prev !=3D idle, we should
not count that period towards rq->avg_idle, for
the same reason we do so when we pulled a task.

I'll add a comment in v3 explaining why idle_stamp
needs to be 0.

--=20
All Rights Reversed.

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