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

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Message-ID: <Zu2gHOv7mqArWXLZ@google.com>
Date: Fri, 20 Sep 2024 16:17:32 +0000
From: Quentin Perret <qperret@...gle.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
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

Hi Vincent,

On Friday 30 Aug 2024 at 15:03:08 (+0200), Vincent Guittot wrote:
> Keep looking for an energy efficient CPU even when the system is
> overutilized and use the CPU returned by feec() if it has been able to find
> one. Otherwise fallback to the default performance and spread mode of the
> scheduler.
> A system can become overutilized for a short time when workers of a
> workqueue wake up for a short background work like vmstat update.
> Continuing to look for a energy efficient CPU will prevent to break the
> power packing of tasks.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2273eecf6086..e46af2416159 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8505,7 +8505,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>  		    cpumask_test_cpu(cpu, p->cpus_ptr))
>  			return cpu;
>  
> -		if (!is_rd_overutilized(this_rq()->rd)) {
> +		if (sched_energy_enabled()) {

As mentioned during LPC, when there is no idle time on a CPU, the
utilization value of the tasks running on it is no longer a good
approximation for how much the tasks want, it becomes an image of how
much CPU time they were given. That is particularly problematic in the
co-scheduling case, but not just.

IOW, when we're OU, the util values are bogus, so using feec() is frankly
wrong IMO. If we don't have a good idea of how long tasks want to run,
the EM just can't help us with anything so we should stay away from it.

I understand how just plain bailing out as we do today is sub-optimal,
but whatever we do to improve on that can't be doing utilization-based
task placement.

Have you considered making the default (non-EAS) wake-up path a little
more reluctant to migrations when EAS is enabled? That should allow us
to maintain a somewhat stable task placement when OU is only transient
(e.g. due to misfit), but without using util values when we really
shouldn't.

Thoughts?

Thanks,
Quentin

>  			new_cpu = find_energy_efficient_cpu(p, prev_cpu);
>  			if (new_cpu >= 0)
>  				return new_cpu;
> -- 
> 2.34.1
> 
>