Re: [PATCH] cgroup/cpuset: Support multiple source/destination cpusets using pids pattern

[Ridong Chen <ridong.chen@linux.dev>]

On 6/4/2026 2:47 AM, Waiman Long wrote:
> On 6/3/26 6:26 AM, Ridong Chen wrote:
>> The current cpuset_can_attach() and cpuset_attach() functions assume task
>> migration is from one source cpuset to one destination cpuset. This
>> can be
>> wrong in several scenarios:
>>   - Moving a multi-threaded process with threads in different cpusets
>>   - Disabling the cpuset controller (many children to one parent)
>>   - Enabling the cpuset controller (one parent to many children)
>>
>> Fix this by adopting the pids subsystem's per-task accounting pattern.
>> In cpuset_can_attach(), use task_cs(task) to get the correct source
>> cpuset
>> for each task (like pids_can_attach uses task_css), adjust
>> nr_deadline_tasks
>> and reserve DL bandwidth per-task, and increment attach_in_progress
>> per-task
>> on the destination cpuset. In cpuset_attach(), handle destination cpuset
>> changes within the task iteration loop.
>>
>> A shared helper cpuset_undo_attach() reverses the per-task operations for
>> both partial rollback in cpuset_can_attach() and full reversal in
>> cpuset_cancel_attach().
>>
>> When multiple source cpusets are detected in can_attach(), set
>> attach_many_sources so that cpuset_attach() forces cpus_updated and
>> mems_updated to true, ensuring all tasks get properly updated regardless
>> of which source cpuset cpuset_attach_old_cs points to.
>>
>> This eliminates the need for nr_migrate_dl_tasks, sum_migrate_dl_bw, and
>> dl_bw_cpu fields in struct cpuset.
>>
>> Fixes: 4ec22e9c5a90 ("cpuset: Enable cpuset controller in default
>> hierarchy")
>> Signed-off-by: Ridong Chen <ridong.chen@linux.dev>
> 
> It is not a problem doing per-task DL BW allocation and eliminating the
> *dl_bw* fields. However, updating nr_deadline_tasks before it is
> committed can be problematic.
> 

Good to hear that.

> nr_deadline_tasks is used in dl_rebuild_rd_accounting() which is called
> by partition_sched_domains_locked(). After the release of cpuset_mutex
> at the end of cpuset_can_attach() and before cpuset_attach() or
> cpuset_cancel_attach() is called, it is possible
> that partition_sched_domains_locked() can be called
> and dl_rebuild_rd_accounting() is not getting the right DL BW accounting
> information. So unless there is a way to confirm that this situation
> cannot happen, we can't change nr_deadline_tasks before the attach is
> commited.
> 

We can keep the nr_migrate_dl_tasks field and update nr_deadline_tasks
once migration is complete. I think this will be much simpler than
fixing the issue using lists.

-- 
Best regards,
Ridong