On Tue, May 25, 2021 at 04:14:18PM +0100, Will Deacon wrote:

 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();
+	cs_mask = task_cs(tsk)->cpus_allowed;
+
+	if (!is_in_v2_mode() || !cpumask_subset(cs_mask, possible_mask))
+		goto unlock; /* select_fallback_rq will try harder */
+
+	do_set_cpus_allowed(tsk, cs_mask);
+unlock:

	if (is_in_v2_mode() && cpumask_subset(cs_mask, possible_mask))
		do_set_cpus_allowed(tsk, cs_mask);

perhaps?