linux-kernel - Re: [PATCH v3] sched/core: Skip user_cpus

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Message-ID: <2vpxlzo6kruo23ljelerqkofybovtrxalinbz56wgpek6j47et@om6jyuyqecog>
Date: Mon, 21 Jul 2025 17:13:40 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Waiman Long <longman@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	Chen Ridong <chenridong@...weicloud.com>, Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH v3] sched/core: Skip user_cpus_ptr masking if no online
 CPU left

On Fri, Jul 18, 2025 at 12:48:56PM -0400, Waiman Long <longman@...hat.com> wrote:
> Chen Ridong reported that cpuset could report a kernel warning for a task
> due to set_cpus_allowed_ptr() returning failure in the corner case that:
> 
> 1) the task used sched_setaffinity(2) to set its CPU affinity mask to
>    be the same as the cpuset.cpus of its cpuset,
> 2) all the CPUs assigned to that cpuset were taken offline, and
> 3) cpuset v1 is in use and the task had to be migrated to the top cpuset.

Does this make sense for cpuset v2 (or no cpuset at all for that matter)?
I'm asking whether this mask modification could only be extracted into
cpuset-v1.c (like cgroup_tranfer_tasks() or a new function)

Thanks,
Michal

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