linux-kernel - Re: [RFC PATCH 4/5] sched/fair: Use EAS also when overutilized

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Message-ID: <CAKfTPtARhtaN=ApYzkYucGp06Gb-m260gKG9hVLkE7BJSVQvQg@mail.gmail.com>
Date: Thu, 3 Oct 2024 15:26:17 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Quentin Perret <qperret@...gle.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, vschneid@...hat.com, lukasz.luba@....com, 
	rafael.j.wysocki@...el.com, linux-kernel@...r.kernel.org, qyousef@...alina.io, 
	hongyan.xia2@....com
Subject: Re: [RFC PATCH 4/5] sched/fair: Use EAS also when overutilized

On Thu, 3 Oct 2024 at 11:52, Quentin Perret <qperret@...gle.com> wrote:
>
> On Thursday 03 Oct 2024 at 10:57:55 (+0200), Vincent Guittot wrote:
> > The current pelt algorithm track actual cpu utilization and can go
> > above cpu capacity (but not above 1024) so a  task utilization can
> > become bigger than a little cpu capacity
>
> Right, the time invariance thing. So yes, I still think that a mix of
> co-scheduling and task migrations (which is likely common in the
> overcommitted state) will cause some CPUs to appear lightly utilized at
> least transiently, hence tricking feec() into thinking it can help when
> it really can't.
>
> > As replied to Lukasz, if you want to discard utilization of a trask
> > you need to check the previous cpu
>
> Please help me out here because I'm still not quite sure what we're
> talking about. Could you please expand a bit?

If you consider that utilization of a task is meaningless because of
CPU overcommitment then you need to check if the prev cpu of the
waking task is/was overcommitted or not the last time the task run in
addition to the overcommitment of the next cpu

>
> Thanks,
> Quentin