linux-kernel - Re: [cgroup/for-6.20 PATCH v2 3/4] cgroup/cpuset: Don't fail cpuset.cpus change in v2

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Message-ID: <gscfzr3lfd2gynec3ci6puz2cyvftrv35udrzzyfi4kjdicfcp@6vdgm645fmmj>
Date: Thu, 8 Jan 2026 20:03:52 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Waiman Long <llong@...hat.com>
Cc: Chen Ridong <chenridong@...weicloud.com>, Tejun Heo <tj@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>, 
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-doc@...r.kernel.org, Sun Shaojie <sunshaojie@...inos.cn>
Subject: Re: [cgroup/for-6.20 PATCH v2 3/4] cgroup/cpuset: Don't fail
 cpuset.cpus change in v2

On Sun, Jan 04, 2026 at 04:48:06PM -0500, Waiman Long <llong@...hat.com> wrote:
> Before the introduction of cpuset partition, writing to cpuset.cpus will
> only fail if the cpu list is invalid like containing CPUs outside of the
> valid cpu range. What I mean by "never-fail" is that if the cpu list is
> valid, the write action should not fail. The rule is not explicitly stated
> in the documentation, but it is a pre-existing behavior which we should try
> to keep to avoid breaking existing applications.

The justification for such behavior is that when the configuration
cannot be satisfied immediately (insufficient resources in ancestors),
the original user's intention should be stored somewhere and if the
conditions higher up the hiearchy possibly change, the intended config
is effected transparently (w/out the need to re-write values by user
again).

So I appreciate that cpuset.cpus.exclusive writes fail early -- for
sibling conflicts -- otherwise the order of creation would need to be
evaluated post hoc.
For illustration:
	a1/cpuset.cpus.exclusive=0,1
	a2/cpuset.cpus.exclusive=1,2
	a3/cpuset.cpus.exclusive=1,3
If this was allowed and a1 was rmdir'd, the (new) resolution of conflict
between a2 and a3 would need to determine which of a2, a3 was created
first.

HTH,
Michal

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