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Subject[PATCH v2] sched/fair: fix enqueue_task_fair warning
Date
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
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