linux-kernel - Re: [PATCH v2] cpuset: relax the overlap check for cgroup-v2

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Message-ID: <4b7znoqq6sdtutcn3jafyrucpqe5jylryvoeooz5ah54vbei4f@wxhsd7gkj3tp>
Date: Fri, 14 Nov 2025 17:15:05 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Sun Shaojie <sunshaojie@...inos.cn>
Cc: llong@...hat.com, chenridong@...weicloud.com, cgroups@...r.kernel.org, 
	hannes@...xchg.org, linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	shuah@...nel.org, tj@...nel.org
Subject: Re: [PATCH v2] cpuset: relax the overlap check for cgroup-v2

On Fri, Nov 14, 2025 at 02:24:48PM +0800, Sun Shaojie <sunshaojie@...inos.cn> wrote:
> The desired outcome is that after step #5, although B1 writes "0-3" to 
> cpuset.cpus, A1 can still remain as "root", and B1 ends up with effective 
> CPUs of 2-3. In summary, We want to avoid A1's invalidation when B1 
> changes its cpuset.cpus. Because cgroup v2 allows the effective CPU mask 
> of a cpuset to differ from its requested mask.

So the new list of reasons why configured cpuset's cpus change are:
- hotplug,
- ancestor's config change,
- stealing by a sibling (new).

IIUC, the patch proposes this behavior:

  echo root >A1.cpuset.partition
  echo 0-1 >A1.cpuset.cpus
  
  echo root >B1.cpuset.partition
  echo 1-2 >B1.cpuset.cpus	# invalidates A1
  
  echo 0-1 >A1.cpuset.cpus	# invalidates B1
  
  ping-pong over CPU 1 ad libitum

I think the right (tm) behavior would be not to depend on the order in
which config is applied to siblings, i.e.

  echo root >A1.cpuset.partition
  echo 0-1 >A1.cpuset.cpus
  
  echo root >B1.cpuset.partition
  echo 1-2 >B1.cpuset.cpus	# invalidates both A1 and B1

  echo 0-1 >A1.cpuset.cpus	# no change anymore

(I hope my example sheds some light on my understanding of the situation
and desired behavior.)

Thanks,
Michal

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