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

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Message-ID: <CAKfTPtCjax48zq3dzuupfrz52Q5TVF=kYSm0t35kT=E4QJGQ7w@mail.gmail.com>
Date: Fri, 8 Nov 2024 10:02:04 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Saravana Kannan <saravanak@...gle.com>
Cc: Ingo Molnar <mingo@...hat.com>, "Peter Zijlstra (Intel)" <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, 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, 8 Nov 2024 at 08:28, Saravana Kannan <saravanak@...gle.com> 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.
>
> The async suspend/resume code calls async_schedule_dev_nocall() to
> queue up a bunch of work. These queued up work seem to be running in
> kworker threads.
>
> However, there have been many times where I see scheduling latency
> (runnable, but not running) of 4.5 ms or higher for a kworker thread
> when there are plenty of idle CPUs.

You are using EAS, aren't you ?
so the energy impact drive the cpu selection not cpu idleness

There is a proposal to change feec to also take into account such case
in addition to the energy impact
https://lore.kernel.org/lkml/64ed0fb8-12ea-4452-9ec2-7ad127b65529@arm.com/T/

I still have to finalize v2

>
> Does async_schedule_dev_nocall() have some weird limitations on where
> they can be run? I know it has some NUMA related stuff, but the Pixel
> 6 doesn't have NUMA. This oddity unnecessarily increases
> suspend/resume latency as it adds up across kworker threads. So, I'd
> appreciate any insights on what might be happening?
>
> If you know how to use perfetto (it's really pretty simple, all you
> need to know is WASD and clicking), here's an example:
> https://ui.perfetto.dev/#!/?s=e20045736e7dfa1e897db6489710061d2495be92
>
> Thanks,
> Saravana