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

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Message-Id: <20251124102024.1768386-1-sunshaojie@kylinos.cn>
Date: Mon, 24 Nov 2025 18:20:24 +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 Sat, 22 Nov 2025 09:33:34, Chen Ridong wrote:
>On 2025/11/21 18:32, Sun Shaojie wrote:
>> 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?
>> 
>
> #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
> mkdir A4
> mkdir A5
> echo "0" > A4/cpuset.cpus
> echo $$ > A4/cgroup.procs
> echo "1" > A5/cpuset.cpus
> echo $$ > A5/cgroup.procs
>
>You might be going to argue that we haven't set the cpus for A4/A5..., yeah, maybe a corner case.
>
>However, it’s common practice to configure a cpuset’s cpus first and then migrate processes—this is
>a typical usage scenario.
>

I'm sorry, I didn't quite understand the point you were trying to make with this example.

If that's the case

     root cgroup
          |
          A1
       / /  \ \
     A2 A3  A4 A5

 #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
 mkdir A4
 mkdir A5
 echo "0" > A4/cpuset.cpus
 echo $$ > A4/cgroup.procs  ->This will return an error because A4's effective CPUs are empty.
 echo "1" > A5/cpuset.cpus
 echo $$ > A5/cgroup.procs  ->This will return an error because A5's effective CPUs are empty.

Even with this patch applied, this result will not change.

>
>> 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?

Thanks,
Sun Shaojie