linux-kernel - [PATCH] sched/fair : fix reordering of enqueue_task

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Date:   Wed,  4 Mar 2020 16:17:48 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, linux-kernel@...r.kernel.org
Cc:     pauld@...hat.com, parth@...ux.ibm.com, valentin.schneider@....com,
        hdanton@...a.com, zhout@...aldi.net,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH] sched/fair : fix reordering of enqueue_task_fair

Even when a cgroup is throttled, the group se of a child cgroup can still
be enqueued and its gse->on_rq stays true. When a task is enqueued on such
child, we still have to update the load_avg and increase h_nr_running of
the throttled cfs. Nevertheless, the 1st for_each_sched_entity loop is
skipped because of gse->on_rq == true and the 2nd loop because the cfs is
throttled whereas we have to update both load_avg with the old
h_nr_running and increment h_nr_running in such case.

Note that the update of load_avg will effectively happen only once in order
to sync with the throttled time. Next call for updating load_avg will stop
early because the clock stays unchanged.

Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fcc968669aea..22d827fc46c3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5432,7 +5432,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 		cfs_rq = cfs_rq_of(se);

 		/* end evaluation on encountering a throttled cfs_rq */
-		if (cfs_rq_throttled(cfs_rq))
+		if (!se->on_rq && cfs_rq_throttled(cfs_rq))
 			goto enqueue_throttle;

 		update_load_avg(cfs_rq, se, UPDATE_TG);
-- 
2.17.1