- set_cpus_allowed() optimization from Mike Kravetz: we can

  set p->thread_info->cpu directly if the task is not running
  and is not on any runqueue.

kernel/sched.c

@@ -1725,7 +1725,15 @@ void set_cpus_allowed(task_t *p, unsigned long new_mask)
 		task_rq_unlock(rq, &flags);
 		goto out;
 	}
-
+	/*
+	 * If the task is not on a runqueue (and not running), then
+	 * it is sufficient to simply update the task's cpu field.
+	 */
+	if (!p->array && (p != rq->curr)) {
+		p->thread_info->cpu = __ffs(p->cpus_allowed);
+		task_rq_unlock(rq, &flags);
+		goto out;
+	}
 	init_MUTEX_LOCKED(&req.sem);
 	req.task = p;
 	list_add(&req.list, &rq->migration_queue);