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

[Chen Ridong <chenridong@huaweicloud.com>]

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.


> 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?
> 


-- 
Best regards,
Ridong