[PATCH v3 for-4.12-fixes 2/2] sched/fair: Fix O(# total cgroups) in load balance path

[Tejun Heo <tj@kernel.org>]

Currently, rq->leaf_cfs_rq_list is a traversal ordered list of all
live cfs_rqs which have ever been active on the CPU; unfortunately,
this makes update_blocked_averages() O(# total cgroups) which isn't
scalable at all.

This shows up as a small CPU consumption and scheduling latency
increase in the load balancing path in systems with CPU controller
enabled across most cgroups.  In an edge case where temporary cgroups
were leaking, this caused the kernel to consume good several tens of
percents of CPU cycles running update_blocked_averages(), each run
taking multiple millisecs.

This patch fixes the issue by taking empty and fully decayed cfs_rqs
off the rq->leaf_cfs_rq_list.

v3: The debug path still uses for_each_leaf_cfs_rq().  Keep it around.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Turner <pjt@google.com>
Cc: Chris Mason <clm@fb.com>
Cc: stable@vger.kernel.org
---
Hello,

The previous patch caused build error if SCHED_DEBUG is set as that
still uses for_each_leaf_cfs_rq().  Patch updated to keep that around.

Thanks.

 kernel/sched/fair.c |   19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -372,6 +372,10 @@ static inline void list_del_leaf_cfs_rq(
 #define for_each_leaf_cfs_rq(rq, cfs_rq) \
 	list_for_each_entry_rcu(cfs_rq, &rq->leaf_cfs_rq_list, leaf_cfs_rq_list)
 
+#define for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos) \
+	list_for_each_entry_safe(cfs_rq, pos, &rq->leaf_cfs_rq_list, \
+				 leaf_cfs_rq_list)
+
 /* Do the two (enqueued) entities belong to the same group ? */
 static inline struct cfs_rq *
 is_same_group(struct sched_entity *se, struct sched_entity *pse)
@@ -466,6 +470,9 @@ static inline void list_del_leaf_cfs_rq(
 #define for_each_leaf_cfs_rq(rq, cfs_rq) \
 		for (cfs_rq = &rq->cfs; cfs_rq; cfs_rq = NULL)
 
+#define for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos) \
+		for (cfs_rq = &rq->cfs; cfs_rq; cfs_rq = NULL)
+
 static inline struct sched_entity *parent_entity(struct sched_entity *se)
 {
 	return NULL;
@@ -6984,7 +6991,7 @@ static void attach_tasks(struct lb_env *
 static void update_blocked_averages(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
-	struct cfs_rq *cfs_rq;
+	struct cfs_rq *cfs_rq, *pos;
 	struct rq_flags rf;
 
 	rq_lock_irqsave(rq, &rf);
@@ -6994,7 +7001,7 @@ static void update_blocked_averages(int
 	 * Iterates the task_group tree in a bottom up fashion, see
 	 * list_add_leaf_cfs_rq() for details.
 	 */
-	for_each_leaf_cfs_rq(rq, cfs_rq) {
+	for_each_leaf_cfs_rq_safe(rq, cfs_rq, pos) {
 		struct sched_entity *se;
 
 		/* throttled entities do not contribute to load */
@@ -7008,6 +7015,14 @@ static void update_blocked_averages(int
 		se = cfs_rq->tg->se[cpu];
 		if (se && !skip_blocked_update(se))
 			update_load_avg(se, 0);
+
+		/*
+		 * There can be a lot of idle CPU cgroups.  Don't let fully
+		 * decayed cfs_rqs linger on the list.
+		 */
+		if (!cfs_rq->load.weight && !cfs_rq->avg.load_sum &&
+		    !cfs_rq->avg.util_sum && !cfs_rq->runnable_load_sum)
+			list_del_leaf_cfs_rq(cfs_rq);
 	}
 	rq_unlock_irqrestore(rq, &rf);
 }