Hello.

On Fri, Mar 17, 2023 at 11:15:07AM -0400, Waiman Long <longman@redhat.com> wrote:
>   * Iterate through each task of @cs updating its cpus_allowed to the
>   * effective cpuset's.  As this function is called with cpuset_rwsem held,
> - * cpuset membership stays stable.
> + * cpuset membership stays stable. For top_cpuset, task_cpu_possible_mask()
> + * is used instead of effective_cpus to make sure all offline CPUs are also
> + * included as hotplug code won't update cpumasks for tasks in top_cpuset.
>   */

On Wed, Mar 15, 2023 at 11:06:20AM +0100, Michal Koutný <mkoutny@suse.com> wrote:
> I see now that it returns offlined cpus to top cpuset's tasks.

I considered only the "base" set change cs->effective_cpus ->
possible_mask. (Apologies for that mistake.)

However, I now read the note about subparts_cpus

>         * effective_cpus contains only onlined CPUs, but subparts_cpus
>         * may have offlined ones.

So if subpart_cpus keeps offlined CPUs, they will be subtracted from
possible_mask and absent in the resulting new_cpus, i.e. undesirable for
the tasks in that cpuset :-/

Michal