linux-kernel - Re: [PATCH v5] cpuset: Avoid invalidating sibling partitions on cpuset.cpus conflict.

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Message-Id: <20251121103217.1661443-1-sunshaojie@kylinos.cn>
Date: Fri, 21 Nov 2025 18:32:17 +0800
From: Sun Shaojie <sunshaojie@...inos.cn>
To: chenridong@...weicloud.com
Cc: cgroups@...r.kernel.org,
	hannes@...xchg.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	llong@...hat.com,
	mkoutny@...e.com,
	shuah@...nel.org,
	sunshaojie@...inos.cn,
	tj@...nel.org
Subject: Re: [PATCH v5] cpuset: Avoid invalidating sibling partitions on cpuset.cpus conflict.

Hi, Ridong,

On Thu, 20 Nov 2025 21:45:16, Chen Ridong wrote:
>On 2025/11/20 21:07, Sun Shaojie wrote:
>> I have carefully considered the scenario where parent effective CPUs are 
>> empty, which corresponds to the following two cases. (After apply this patch).
>> 
>>    root cgroup
>>         |
>>        A1
>>       /  \
>>     A2    A3
>> 
>> Case 1:
>>  Step:
>>  #1> echo "0-1" > A1/cpuset.cpus
>>  #2> echo "root" > A1/cpuset.cpus.partition
>>  #3> echo "0-1" > A2/cpuset.cpus
>>  #4> echo "root" > A2/cpuset.cpus.partition
>> 
>>  After step #4, 
>> 
>>                 |      A1      |      A2      |      A3      |
>>  cpus_allowed   | 0-1          | 0-1          |              |
>>  effective_cpus |              | 0-1          |              |
>>  prstate        | root         | root         | member       |
>> 
>>  After step #4, A3's effective CPUs is empty.
>> 
>
>That may be a corner case is unexpected.
>
>>  #5> echo "0-1" > A3/cpuset.cpus
>> 
>
>If we create subdirectories (e.g., A4, A5, ...) under the A1 cpuset and then configure cpuset.cpus
>for A1 (a common usage scenario), processes can no longer be migrated into these subdirectories (A4,
>A5, ...) afterward. However, prior to your patch, this migration was allowed.

Are you referring to creating subdirectories (A4, A5, ...) after step #4? 
And what parameters should be configured for A1's cpuset.cpus?
Could you provide a specific example?

Additionally, processes cannot be migrated into a cgroup whose 
cpuset.cpus.effective is empty. However, this patch does not modify this behavior.

So why does applying this patch enable such migration?

>>  After step #5,
>> 
>>                 |      A1      |      A2      |      A3      |
>>  cpus_allowed   | 0-1          | 0-1          | 0-1          |
>>  effective_cpus |              | 0-1          |              |
>>  prstate        | root         | root         | member       |
>> 


Thanks,
Sun Shaojie