linux-kernel - Re: [PATCH v2 03/12] sched: Optimize ttwu() / select_task

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Message-ID: <jitpfox6tlodcrpm233xifjlxdcuavgo5loxorwhwnhpvvrlvg@qxca4uarce3q>
Date: Mon, 14 Jul 2025 23:59:51 +0100
From: Mel Gorman <mgorman@...hsingularity.net>
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: [PATCH v2 03/12] sched: Optimize ttwu() / select_task_rq()

On Wed, Jul 02, 2025 at 01:49:27PM +0200, Peter Zijlstra wrote:
> Optimize ttwu() by pushing select_idle_siblings() up above waiting for
> on_cpu(). This allows making use of the cycles otherwise spend waiting
> to search for an idle CPU.
> 
> One little detail is that since the task we're looking for an idle CPU
> for might still be on the CPU, that CPU won't report as running the
> idle task, and thus won't find his own CPU idle, even when it is.
> 
> To compensate, remove the 'rq->curr == rq->idle' condition from
> idle_cpu() -- it doesn't really make sense anyway.
> 
> Additionally, Chris found (concurrently) that perf-c2c reported that
> test as being a cache-miss monster.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20250520101727.620602459@infradead.org

*facepalm*

Acked-by: Mel Gorman <mgorman@...hsingularity.net>

-- 
Mel Gorman
SUSE Labs