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: <6eedf67b-3538-4fd1-903b-b7d8db4ff43d@redhat.com>
Date: Sun, 4 Jan 2026 16:48:06 -0500
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, Tejun Heo <tj@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Koutný
 <mkoutny@...e.com>, Jonathan Corbet <corbet@....net>,
 Shuah Khan <shuah@...nel.org>
Cc: 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 1/4/26 2:09 AM, Chen Ridong wrote:
>
> On 2026/1/2 3:15, Waiman Long wrote:
>> Commit fe8cd2736e75 ("cgroup/cpuset: Delay setting of CS_CPU_EXCLUSIVE
>> until valid partition") introduced a new check to disallow the setting
>> of a new cpuset.cpus.exclusive value that is a superset of a sibling's
>> cpuset.cpus value so that there will at least be one CPU left in the
>> sibling in case the cpuset becomes a valid partition root. This new
>> check does have the side effect of failing a cpuset.cpus change that
>> make it a subset of a sibling's cpuset.cpus.exclusive value.
>>
>> With v2, users are supposed to be allowed to set whatever value they
>> want in cpuset.cpus without failure. To maintain this rule, the check
>> is now restricted to only when cpuset.cpus.exclusive is being changed
>> not when cpuset.cpus is changed.
>>
> Hi, Longman,
>
> You've emphasized that modifying cpuset.cpus should never fail. While I haven't found this
> explicitly documented. Should we add it?
>
> More importantly, does this mean the "never fail" rule has higher priority than the exclusive CPU
> constraints? This seems to be the underlying assumption in this patch.

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 exclusive CPU constraint does not apply to cpuset.cpus. It only 
applies when setting cpuset.cpus.exclusive wrt to other 
cpuset.cpus.exclusive* in sibling cpusets. So I will not say one has 
higher priority than the other.

Cheers,
Longman