[PATCH v2] sched/fair: fix enqueue_task_fair warning

[Vincent Guittot <vincent.guittot@linaro.org>]

When a cfs rq is throttled, the latter and its child are removed from the
leaf list but their nr_running is not changed which includes staying higher
than 1. When a task is enqueued in this throttled branch, the cfs rqs must
be added back in order to ensure correct ordering in the list but this can
only happens if nr_running == 1.
When cfs bandwidth is used, we call unconditionnaly list_add_leaf_cfs_rq()
when enqueuing an entity to make sure that the complete branch will be
added.

Similarly unthrottle_cfs_rq() can stop adding cfs in the list when a parent
is throttled. Iterate the remaining entity to ensure that the complete
branch will be added in the list.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: stable@vger.kernel.org #v5.1+
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---

Change since v1:
- added the iteration at the end of unthrottle_cfs_rq() as Dietmar
  reported use case which triggered assert_list_leaf_cfs_rq()

 kernel/sched/fair.c | 26 ++++++++++++++++++++++----
 1 file changed, 22 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fcc968669aea..ae50a09cbead 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4117,6 +4117,7 @@ static inline void check_schedstat_required(void)
 #endif
 }
 
+static inline bool cfs_bandwidth_used(void);
 
 /*
  * MIGRATION
@@ -4195,10 +4196,16 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 		__enqueue_entity(cfs_rq, se);
 	se->on_rq = 1;
 
-	if (cfs_rq->nr_running == 1) {
+	/*
+	 * When bandwidth control is enabled, cfs might have been removed
+	 * because of a parent been throttled but cfs->nr_running > 1. Try to
+	 * add it unconditionnally.
+	 */
+	if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
 		list_add_leaf_cfs_rq(cfs_rq);
+
+	if (cfs_rq->nr_running == 1)
 		check_enqueue_throttle(cfs_rq);
-	}
 }
 
 static void __clear_buddies_last(struct sched_entity *se)
@@ -4779,11 +4786,22 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 			break;
 	}
 
-	assert_list_leaf_cfs_rq(rq);
-
 	if (!se)
 		add_nr_running(rq, task_delta);
 
+	/*
+	 * The cfs_rq_throttled() breaks in the above iteration can result in
+	 * incomplete leaf list maintenance, resulting in triggering the
+	 * assertion below.
+	 */
+	for_each_sched_entity(se) {
+		cfs_rq = cfs_rq_of(se);
+
+		list_add_leaf_cfs_rq(cfs_rq);
+	}
+
+	assert_list_leaf_cfs_rq(rq);
+
 	/* Determine whether we need to wake up potentially idle CPU: */
 	if (rq->curr == rq->idle && rq->cfs.nr_running)
 		resched_curr(rq);
-- 
2.17.1