linux-kernel - Re: [PATCH] sched/fair: check for idle core

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Date:   Wed, 21 Oct 2020 17:23:14 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Julia Lawall <julia.lawall@...ia.fr>
Cc:     Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...hat.com>,
        kernel-janitors@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Gilles Muller <Gilles.Muller@...ia.fr>
Subject: Re: [PATCH] sched/fair: check for idle core

On Wed, 21 Oct 2020 at 17:18, Julia Lawall <julia.lawall@...ia.fr> wrote:
>
>
>
> On Wed, 21 Oct 2020, Mel Gorman wrote:
>
> > On Wed, Oct 21, 2020 at 03:24:48PM +0200, Julia Lawall wrote:
> > > > I worry it's overkill because prev is always used if it is idle even
> > > > if it is on a node remote to the waker. It cuts off the option of a
> > > > wakee moving to a CPU local to the waker which is not equivalent to the
> > > > original behaviour.
> > >
> > > But it is equal to the original behavior in the idle prev case if you go
> > > back to the runnable load average days...
> > >
> >
> > It is similar but it misses the sync treatment and sd->imbalance_pct part of
> > wake_affine_weight which has unpredictable consequences. The data
> > available is only on the fully utilised case.
>
> OK, what if my patch were:
>
> @@ -5800,6 +5800,9 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
>         if (sync && cpu_rq(this_cpu)->nr_running == 1)
>                 return this_cpu;
>
> +       if (!sync && available_idle_cpu(prev_cpu))
> +               return prev_cpu;
> +

this is not useful because when prev_cpu is idle, its runnable_avg was
null so the only
way for this_cpu to be selected by wake_affine_weight is to be null
too which is not really
possible when sync is set because sync is used to say, the running
task on this cpu
is about to sleep

>         return nr_cpumask_bits;
>  }
>
> The sd->imbalance_pct part would have previously been a multiplication by
> 0, so it doesn't need to be taken into account.
>
> julia
>
> >
> > > The problem seems impossible to solve, because there is no way to know by
> > > looking only at prev and this whether the thread would prefer to stay
> > > where it was or go to the waker.
> > >
> >
> > Yes, this is definitely true. Looking at prev_cpu and this_cpu is a
> > crude approximation and the path is heavily limited in terms of how
> > clever it can be.
> >
> > --
> > Mel Gorman
> > SUSE Labs
> >