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

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Message-ID: <alpine.DEB.2.22.394.2010211714300.8475@hadrien>
Date:   Wed, 21 Oct 2020 17:18:00 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Mel Gorman <mgorman@...e.de>
cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        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@...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, 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;
+
 	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
>