sched/fair: Optimize update_blocked_averages()

Removing a cfs_rq from rq->leaf_cfs_rq_list can break the parent/child
ordering of the list when it will be added back. In order to remove an
empty and fully decayed cfs_rq, we must remove its children too, so they
will be added back in the right order next time.

With a normal decay of PELT, a parent will be empty and fully decayed
if all children are empty and fully decayed too. In such a case, we just
have to ensure that the whole branch will be added when a new task is
enqueued. This is default behavior since :

  commit f678331 ("sched/fair: Fix insertion in rq->leaf_cfs_rq_list")

In case of throttling, the PELT of throttled cfs_rq will not be updated
whereas the parent will. This breaks the assumption made above unless we
remove the children of a cfs_rq that is throttled. Then, they will be
added back when unthrottled and a sched_entity will be enqueued.

As throttled cfs_rq are now removed from the list, we can remove the
associated test in update_blocked_averages().

Signed-off-by: Vincent Guittot <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: vingu-linaro

kernel/sched/fair.c

@@ -346,6 +346,18 @@ static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
 static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
 {
 	if (cfs_rq->on_list) {
+		struct rq *rq = rq_of(cfs_rq);
+
+		/*
+		 * With cfs_rq being unthrottled/throttled during an enqueue,
+		 * it can happen the tmp_alone_branch points the a leaf that
+		 * we finally want to del. In this case, tmp_alone_branch moves
+		 * to the prev element but it will point to rq->leaf_cfs_rq_list
+		 * at the end of the enqueue.
+		 */
+		if (rq->tmp_alone_branch == &cfs_rq->leaf_cfs_rq_list)
+			rq->tmp_alone_branch = cfs_rq->leaf_cfs_rq_list.prev;
+
 		list_del_rcu(&cfs_rq->leaf_cfs_rq_list);
 		cfs_rq->on_list = 0;
 	}
@@ -4438,6 +4450,10 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
 		/* adjust cfs_rq_clock_task() */
 		cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
 					     cfs_rq->throttled_clock_task;
+
+		/* Add cfs_rq with already running entity in the list */
+		if (cfs_rq->nr_running >= 1)
+			list_add_leaf_cfs_rq(cfs_rq);
 	}
 
 	return 0;
@@ -4449,8 +4465,10 @@ static int tg_throttle_down(struct task_group *tg, void *data)
 	struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
 
 	/* group is entering throttled state, stop time */
-	if (!cfs_rq->throttle_count)
+	if (!cfs_rq->throttle_count) {
 		cfs_rq->throttled_clock_task = rq_clock_task(rq);
+		list_del_leaf_cfs_rq(cfs_rq);
+	}
 	cfs_rq->throttle_count++;
 
 	return 0;
@@ -4553,6 +4571,8 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 			break;
 	}
 
+	assert_list_leaf_cfs_rq(rq);
+
 	if (!se)
 		add_nr_running(rq, task_delta);
 
@@ -7700,10 +7720,6 @@ static void update_blocked_averages(int cpu)
 	for_each_leaf_cfs_rq(rq, cfs_rq) {
 		struct sched_entity *se;
 
-		/* throttled entities do not contribute to load */
-		if (throttled_hierarchy(cfs_rq))
-			continue;
-
 		if (update_cfs_rq_load_avg(cfs_rq_clock_pelt(cfs_rq), cfs_rq))
 			update_tg_load_avg(cfs_rq, 0);