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

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Message-ID: <20201021131827.GF32041@suse.de>
Date:   Wed, 21 Oct 2020 14:18:27 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Julia Lawall <julia.lawall@...ia.fr>
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@...ia.fr
Subject: Re: [PATCH] sched/fair: check for idle core

On Wed, Oct 21, 2020 at 02:56:06PM +0200, Julia Lawall wrote:
> 
> 
> On Wed, 21 Oct 2020, Mel Gorman wrote:
> 
> > 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.
> 
> My goal was to restore the previous behavior, when runnable load was used.
> The patch removing the use of runnable load (11f10e5420f6) presented it
> basically as that load balancing was using it, so wakeup should use it
> too, and any way it didn't matter because idle CPUS were checked for
> anyway.
> 

Which is fair.

> Is your point of view that the proposed change is overkill?  Or is it that
> the original behavior was not desirable?
> 

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.

-- 
Mel Gorman
SUSE Labs