Re: [PATCH v2] cgroup/cpuset: Don't filter offline CPUs in cpuset_cpus_allowed() for top cpuset tasks

[Tejun Heo]

On Sun, Feb 05, 2023 at 10:48:53PM -0500, Waiman Long wrote:
> Since commit 8f9ea86fdf99 ("sched: Always preserve the user
> requested cpumask"), relax_compatible_cpus_allowed_ptr() is calling
> __sched_setaffinity() unconditionally. This helps to expose a bug in
> the current cpuset hotplug code where the cpumasks of the tasks in
> the top cpuset are not updated at all when some CPUs become online or
> offline. It is likely caused by the fact that some of the tasks in the
> top cpuset, like percpu kthreads, cannot have their cpu affinity changed.
> 
> One way to reproduce this as suggested by Peter is:
>  - boot machine
>  - offline all CPUs except one
>  - taskset -p ffffffff $$
>  - online all CPUs
> 
> Fix this by allowing cpuset_cpus_allowed() to return a wider mask that
> includes offline CPUs for those tasks that are in the top cpuset. For
> tasks not in the top cpuset, the old rule applies and only online CPUs
> will be returned in the mask since hotplug events will update their
> cpumasks accordingly.
> 
> Fixes: 8f9ea86fdf99 ("sched: Always preserve the user requested cpumask")
> Reported-by: Will Deacon <will@xxxxxxxxxx>
> Originally-from: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Tested-by: Will Deacon <will@xxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>

Applied to cgroup/for-6.2-fixes.

Thanks.

-- 
tejun