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

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Date:   Wed, 21 Oct 2020 13:47:00 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Julia Lawall <Julia.Lawall@...ia.fr>,
        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@...ia.fr
Subject: Re: [PATCH] sched/fair: check for idle core

On Wed, Oct 21, 2020 at 02:25:32PM +0200, Vincent Guittot wrote:
> > I see Vincent already agreed with the patch so I could be wrong.  Vincent,
> > did I miss something stupid?
> 
> This patch fixes the problem that we don't favor anymore the prev_cpu when it is idle since
> commit 11f10e5420f6ce because load is not null when cpu is idle whereas runnable_load was
> And this is important because this will then decide in which LLC we will looks for a cpu
> 

Ok, that is understandable but I'm still concerned that the fix simply
trades one problem for another by leaving related tasks remote to each
other and increasing cache misses and remote data accesses.

wake_affine_weight is a giant pain because really we don't care about the
load on the waker CPU or its available, we care about whether it has idle
siblings that can be found quickly. As tempting as ripping it out is,
it never happened because sometimes it makes the right decision.

-- 
Mel Gorman
SUSE Labs