linux-kernel - Re: [PATCH v2] sched,fair: skip newidle

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Date:   Tue, 20 Apr 2021 17:44:44 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Kernel Team <kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [PATCH v2] sched,fair: skip newidle_balance if a wakeup is pending

On Tue, 20 Apr 2021 at 17:20, Rik van Riel <riel@...riel.com> wrote:
>
> On Tue, 2021-04-20 at 11:04 +0200, Vincent Guittot wrote:
> > On Mon, 19 Apr 2021 at 18:51, Rik van Riel <riel@...riel.com> wrote:
> > >
> > > @@ -10688,7 +10697,7 @@ static int newidle_balance(struct rq
> > > *this_rq, struct rq_flags *rf)
> > >         if (this_rq->nr_running != this_rq->cfs.h_nr_running)
> > >                 pulled_task = -1;
> > >
> > > -       if (pulled_task)
> > > +       if (pulled_task || this_rq->ttwu_pending)
> >
> > 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()
> >
> > >                 this_rq->idle_stamp = 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.

Not sure that this is really wrong because it ends up scheduling the
idle task which is immediately preempted. But the preemption happened
in the idle task, isn't it ?

>
> 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 != idle, we should
> not count that period towards rq->avg_idle, for
> the same reason we do so when we pulled a task.

As mentioned above, we have effectively schedule the idle task in your
case whereas we don't in the other cases

IIUC, your problem comes from rq->avg_idle decreasing a lot in such
cases. And because rq->avg_idle is used to decide if you have time to
run newlyidle_balance,you skip it more often.

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

Yes please.

>
> --
> All Rights Reversed.