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

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Message-ID: <9bd275be-45df-47f3-9be3-f7e1458815a4@redhat.com>
Date: Mon, 21 Jul 2025 11:28:15 -0400
From: Waiman Long <llong@...hat.com>
To: Michal Koutný <mkoutny@...e.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 7/21/25 11:13 AM, Michal Koutný wrote:
> 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)

This corner case as specified in Chen Ridong's patch only happens with a 
cpuset v1 environment, but it is still the case that the default cpu 
affinity of the root cgroup (with or without CONFIG_CGROUPS) will 
include offline CPUs, if present. So it still make senses to skip the 
sched_setaffinity() setting if there is no online CPU left, though it 
will be much harder to have such a condition without using cpuset v1.

Cheers,
Longman