linux-kernel - [PATCH 1/2] sched/fair: optimization of update_blocked

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Date:   Wed,  6 Feb 2019 17:14:21 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org
Cc:     tj@...nel.org, sargun@...gun.me, xiexiuqi@...wei.com,
        xiezhipeng1@...wei.com, torvalds@...ux-foundation.org,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH 1/2] sched/fair: optimization of 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 f6783319737f ("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 <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ffd1ae7..badf8173 100644
--- a/kernel/sched/fair.c
+++ b/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;
@@ -7699,10 +7717,6 @@ 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);
 
-- 
2.7.4