Re: [PATCH v3] sched/core: Skip user_cpus_ptr masking if no online CPU left

[Waiman Long <llong@redhat.com>]

On 8/26/25 10:25 AM, Michal Koutný wrote:
> Hi.
>
> I had a look after a while (thanks for reminders Ridong).
>
> On Mon, Jul 21, 2025 at 11:28:15AM -0400, Waiman Long <llong@redhat.com> wrote:
>> This corner case as specified in Chen Ridong's patch only happens with a
>> cpuset v1 environment, but it is still the case that the default cpu
>> affinity of the root cgroup (with or without CONFIG_CGROUPS) will include
>> offline CPUs, if present.
> IIUC, the generic sched_setaffinity(2) is ready for that, simply
> returning an EINVAL.

The modified code will not be executed when called from 
sched_setaffiity() as the SCA_USER flag will be set.

In the described scenario, sched_setaffinity() was called without 
failure as the request was valid at the time.

>
>> So it still make senses to skip the sched_setaffinity() setting if
>> there is no online CPU left, though it will be much harder to have
>> such a condition without using cpuset v1.
> That sounds like there'd be no issue without cpuset v1 and the source of
> the warning has quite a telling comment:
>
> 	 * fail.  TODO: have a better way to handle failure here
> 	 */
> 	WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
>
> The trouble is that this is from cpuset_attach() (cgroup_subsys.attach)
> where no errors are expected. So I'd say the place for the check should
> be earlier in cpuset_can_attach() [1]. I'm not sure if that's universally
> immune against cpu offlining but it'd be sufficient for the reported
> sequential offlining.

Cpuset1 has no concept of effective cpumask  that excludes offline CPUs 
unless "cpuset_v2_mode" mount option is used. So when the cpuset has no 
CPU left, it will force migrate the tasks to its parent and the 
__set_cpus_allowed_ptr() function will be invoked. The parent will 
likely have those offline CPUs in their cpus_allowed list and 
__set_cpus_allowed_ptr_locked() will be called with only the offline 
CPUs causing the warning. Migrating to the top_cpuset is probably not 
needed to illustrate the problem.

Cheers,
Longman

> HTH,
> Michal
>
> [1] Although the error propagates, it ends up without recovery in
> remove_tasks_in_empty_cpuset() "only" as an error message. But that's
> likely all what can be done in this workfn context -- it's better than
> silently skipping the migration as consequence of this patch.