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From:   Vincent Guittot <vincent.guittot@linaro.org>
To:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, linux-kernel@vger.kernel.org,
        borntraeger@de.ibm.com
Cc:     Vincent Guittot <vincent.guittot@linaro.org>,
        stable@vger.kernel.org
Subject: [PATCH] sched/fair: fix enqueue_task_fair warning
Date:   Thu,  5 Mar 2020 18:29:21 +0100
Message-Id: <20200305172921.22743-1-vincent.guittot@linaro.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: stable@vger.kernel.org #v5.1+
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fcc968669aea..bdc5bb72ab31 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)
-- 
2.17.1