On 07/06/21 18:20, Will Deacon wrote:

On Fri, Jun 04, 2021 at 06:11:03PM +0100, Valentin Schneider wrote:

quoted

On 02/06/21 17:47, Will Deacon wrote:

quoted

@@ -3322,9 +3322,13 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)

 void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
 {
+	const struct cpumask *cs_mask;
+	const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
+
      rcu_read_lock();
-	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
-		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
+	cs_mask = task_cs(tsk)->cpus_allowed;
+	if (is_in_v2_mode() && cpumask_subset(cs_mask, possible_mask))
+		do_set_cpus_allowed(tsk, cs_mask);

Since the task will still go through the is_cpu_allowed() loop in
select_fallback_rq() after this, is the subset check actually required
here?

Yes, I think it's needed. do_set_cpus_allowed() doesn't do any checking
against the task_cpu_possible_mask, so if we returned to
select_fallback_rq() with a mask containing a mixture of 32-bit-capable and
64-bit-only CPUs then we'd end up setting an affinity mask for a 32-bit
task which contains 64-bit-only cores.

Once again, you're right :-)