linux-kernel - Re: [RFC PATCH] sched/fair: Remove sched_idle_cpu() usages in select_task_rq

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Message-ID: <87212818-1f59-45e4-9a51-dca62ddb9633@arm.com>
Date: Mon, 18 Aug 2025 14:24:27 +0100
From: Christian Loehle <christian.loehle@....com>
To: Chengming Zhou <chengming.zhou@...ux.dev>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com
Cc: linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/fair: Remove sched_idle_cpu() usages in
 select_task_rq_fair()

On 8/18/25 13:47, Chengming Zhou wrote:
> These sched_idle_cpu() considerations in select_task_rq_fair() is based
> on an assumption that the wakee task can pick a cpu running sched_idle
> task and preempt it to run, faster than picking an idle cpu to preempt
> the idle task.
> 
> This assumption is correct, but it also brings some problems:
> 
> 1. work conservation: Often sched_idle tasks are also picking the cpu
> which is already running sched_idle task, instead of utilizing a real
> idle cpu, so work conservation is somewhat broken.
> 
> 2. sched_idle group: This sched_idle_cpu() is just not correct with
> sched_idle group running. Look a simple example below.
> 
> 		root
> 	/		\
> 	kubepods	system
> 	/	\
> burstable	besteffort
> 		(cpu.idle == 1)
> 
> When a sched_idle cpu is just running tasks from besteffort group,
> sched_idle_cpu() will return true in this case, but this cpu pick
> is bad for wakee task from system group. Because the system group
> has lower weight than kubepods, work conservation is somewhat
> broken too.
> 
> In a nutshell, sched_idle_cpu() should consider the wakee task group's
> relationship with sched_idle tasks running on the cpu.
> 
> Obviously, it's hard to do so. This patch chooses the simple approach
> to remove all sched_idle_cpu() considerations in select_task_rq_fair()
> to bring back work conservation in these cases.

OTOH sched_idle_cpu() CPUs are guaranteed to not be in an idle state and
potentially already have DVFS on some higher level...