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: <85f4bca2-e355-49ce-81e9-3b8080082545@redhat.com>
Date: Sun, 4 Jan 2026 22:59:38 -0500
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, Waiman Long <llong@...hat.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 8:35 PM, Chen Ridong wrote:
>
> On 2026/1/5 5:48, Waiman Long wrote:
>> 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.
>>
> There are two conditions that can cause a cpuset.cpus write operation to fail: ENOSPC (No space left
> on device) and EBUSY.
>
> I just want to ensure the behavior aligns with our design intent.
>
> Consider this example:
>
> # cd /sys/fs/cgroup/
> # mkdir test
> # echo 1 > test/cpuset.cpus
> # echo $$ > test/cgroup.procs
> # echo 0 > /sys/devices/system/cpu/cpu1/online
> # echo > test/cpuset.cpus
> -bash: echo: write error: No space left on device
>
> In cgroups v2, if the test cgroup becomes empty, it could inherit the parent's effective CPUs. My
> question is: Should we still fail to clear cpuset.cpus (returning an error) when the cgroup is
> populated?

Good catch. This error is for v1. It shouldn't apply for v2. Yes, I 
think we should fix that for v2.

Cheers,
Longman