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

[Waiman Long <longman@redhat.com>]

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.

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.

Cheers,
Longman