linux-kernel - Re: Very high scheduling delay with plenty of idle CPUs

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Message-ID: <CAGETcx-CvWVc=TP5OmUL_iF7fSb1awJB1G8NghM1q_6dYKXkQQ@mail.gmail.com>
Date: Sat, 9 Nov 2024 21:49:42 -0800
From: Saravana Kannan <saravanak@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Steven Rostedt <rostedt@...dmis.org>, Benjamin Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, LKML <linux-kernel@...r.kernel.org>, 
	wuyun.abel@...edance.com, youssefesmat@...omium.org, 
	Thomas Gleixner <tglx@...utronix.de>, efault@....de, 
	K Prateek Nayak <kprateek.nayak@....com>, John Stultz <jstultz@...gle.com>, 
	Vincent Palomares <paillon@...gle.com>
Subject: Re: Very high scheduling delay with plenty of idle CPUs

On Fri, Nov 8, 2024 at 12:31 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Nov 07, 2024 at 11:28:07PM -0800, Saravana Kannan wrote:
> > Hi scheduler folks,
> >
> > I'm running into some weird scheduling issues when testing non-sched
> > changes on a Pixel 6 that's running close to 6.12-rc5. I'm not sure if
> > this is an issue in earlier kernel versions or not.
> >
>
> It's a bit unfortunate you don't have a known good kernel there. Anyway,
> one thing that recently came up is that DELAY_DEQUEUE can cause some
> delays, specifically it can inhibit wakeup migration.

I disabled DELAY_DEQUEUE and I'm still seeing preemptions or
scheduling latency (after wakeup) when there are plenty of CPUs even
within the same cluster/frequency domain.

Can we tell the scheduler to just spread out all the tasks during
suspend/resume? Doesn't make a lot of sense to try and save power
during a suspend/resume. It's almost always cheaper/better to do those
quickly.

-Saravana


-Saravana

>
> You can either test with that feature turned off, or apply something
> like the following patch:
>
>   https://lkml.kernel.org/r/20241106135346.GL24862@noisy.programming.kicks-ass.net