sched: Optimize migration_cpu_stop()

When the purpose of migration_cpu_stop() is to migrate the task to
'any' valid CPU, don't migrate the task when it's already running on a
valid CPU.

Fixes: 6d337ea ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Cc: stable@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224131355.569238629@infradead.org

kernel/sched/core.c

@@ -1936,14 +1936,25 @@ static int migration_cpu_stop(void *data)
 			complete = true;
 		}
 
-		if (dest_cpu < 0)
+		if (dest_cpu < 0) {
+			if (cpumask_test_cpu(task_cpu(p), &p->cpus_mask))
+				goto out;
+
 			dest_cpu = cpumask_any_distribute(&p->cpus_mask);
+		}
 
 		if (task_on_rq_queued(p))
 			rq = __migrate_task(rq, &rf, p, dest_cpu);
 		else
 			p->wake_cpu = dest_cpu;
 
+		/*
+		 * XXX __migrate_task() can fail, at which point we might end
+		 * up running on a dodgy CPU, AFAICT this can only happen
+		 * during CPU hotplug, at which point we'll get pushed out
+		 * anyway, so it's probably not a big deal.
+		 */
+
 	} else if (pending) {
 		/*
 		 * This happens when we get migrated between migrate_enable()'s