[RFC PATCH 7/7] sched/fair: Make sure cfs_rq has enough runtime_remaining on unthrottle path

[Aaron Lu]

It's possible unthrottle_cfs_rq() is called with !runtime_remaining
due to things like user changed quota setting(see tg_set_cfs_bandwidth())
or async unthrottled us with a positive runtime_remaining but other still
running entities consumed those runtime before we reach there.

Anyway, we can't unthrottle this cfs_rq without any runtime remaining
because task enqueue during unthrottle can immediately trigger a throttle
by check_enqueue_throttle(), which should never happen.

Signed-off-by: Aaron Lu <ziqianlu@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index be96f7d32998c..d646451d617c1 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6058,6 +6058,19 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
 	struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
 	struct sched_entity *se = cfs_rq->tg->se[cpu_of(rq)];

+	/*
+	 * It's possible we are called with !runtime_remaining due to things
+	 * like user changed quota setting(see tg_set_cfs_bandwidth()) or async
+	 * unthrottled us with a positive runtime_remaining but other still
+	 * running entities consumed those runtime before we reach here.
+	 *
+	 * Anyway, we can't unthrottle this cfs_rq without any runtime remaining
+	 * because any enqueue below will immediately trigger a throttle, which
+	 * is not supposed to happen on unthrottle path.
+	 */
+	if (cfs_rq->runtime_enabled && !cfs_rq->runtime_remaining)
+		return;
+
 	cfs_rq->throttled = 0;

 	update_rq_clock(rq);
-- 
2.39.5