linux-kernel - Re: [RFC][PATCH 2/5] sched: Optimize ttwu() / select_task

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Message-ID: <b638eb73e6da733afd6eb758cc144bf119e1b600.camel@gmx.de>
Date: Fri, 13 Jun 2025 12:20:56 +0200
From: Mike Galbraith <efault@....de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org, 
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de,  vschneid@...hat.com, clm@...a.com,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/5] sched: Optimize ttwu() / select_task_rq()

On Fri, 2025-06-13 at 11:40 +0200, Peter Zijlstra wrote:
> On Mon, Jun 09, 2025 at 07:01:47AM +0200, Mike Galbraith wrote:
> 
> Right; so the problem being that we can race with
> migrate_disable_switch().

Yeah.  Most of the time when we do fallback saves us, but we can and do
zip past it, and that turns box various shades of sad.

> 
> Does something like this help?

It surely will, but I'll testdrive it.  No news is good news.

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3593,7 +3593,7 @@ int select_task_rq(struct task_struct *p
>  		cpu = p->sched_class->select_task_rq(p, cpu,
> *wake_flags);
>  		*wake_flags |= WF_RQ_SELECTED;
>  	} else {
> -		cpu = cpumask_any(p->cpus_ptr);
> +		cpu = task_cpu(p);
>  	}
>  
>  	/*