cgroup/cpuset: Extend kthread_is_per_cpu() check to all PF_NO_SETAFFINITY tasks

JIRA: https://issues.redhat.com/browse/RHEL-107751

commit 39b5ef7
Author: Waiman Long <longman@redhat.com>
Date:   Thu, 8 May 2025 15:24:13 -0400

    cgroup/cpuset: Extend kthread_is_per_cpu() check to all PF_NO_SETAFFINITY tasks

    Commit ec5fbdf ("cgroup/cpuset: Enable update_tasks_cpumask()
    on top_cpuset") enabled us to pull CPUs dedicated to child partitions
    from tasks in top_cpuset by ignoring per cpu kthreads. However, there
    can be other kthreads that are not per cpu but have PF_NO_SETAFFINITY
    flag set to indicate that we shouldn't mess with their CPU affinity.
    For other kthreads, their affinity will be changed to skip CPUs dedicated
    to child partitions whether it is an isolating or a scheduling one.

    As all the per cpu kthreads have PF_NO_SETAFFINITY set, the
    PF_NO_SETAFFINITY tasks are essentially a superset of per cpu kthreads.
    Fix this issue by dropping the kthread_is_per_cpu() check and checking
    the PF_NO_SETAFFINITY flag instead.

    Fixes: ec5fbdf ("cgroup/cpuset: Enable update_tasks_cpumask() on top_cpuset")
    Signed-off-by: Waiman Long <longman@redhat.com>
    Acked-by: Frederic Weisbecker <frederic@kernel.org>
    Signed-off-by: Tejun Heo <tj@kernel.org>

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

kernel/cgroup/cpuset.c

@@ -1130,9 +1130,11 @@ void cpuset_update_tasks_cpumask(struct cpuset *cs, struct cpumask *new_cpus)
 
 		if (top_cs) {
 			/*
-			 * Percpu kthreads in top_cpuset are ignored
+			 * PF_NO_SETAFFINITY tasks are ignored.
+			 * All per cpu kthreads should have PF_NO_SETAFFINITY
+			 * flag set, see kthread_set_per_cpu().
 			 */
-			if (kthread_is_per_cpu(task))
+			if (task->flags & PF_NO_SETAFFINITY)
 				continue;
 			cpumask_andnot(new_cpus, possible_mask, subpartitions_cpus);
 		} else {