linux-kernel - Re: [PATCH v3] sched/fair: Fix enqueue_task

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Message-ID: <20200512135222.GC2201@lorien.usersys.redhat.com>
Date:   Tue, 12 May 2020 09:52:22 -0400
From:   Phil Auld <pauld@...hat.com>
To:     peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        vincent.guittot@...aro.org, juri.lelli@...hat.com,
        zohooouoto@...o.com.cn, dietmar.eggemann@....com
Subject: Re: [PATCH v3] sched/fair: Fix enqueue_task_fair warning some more

sched/fair: Fix enqueue_task_fair warning some more

The recent patch, fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
did not fully resolve the issues with the rq->tmp_alone_branch !=
&rq->leaf_cfs_rq_list warning in enqueue_task_fair. There is a case where
the first for_each_sched_entity loop exits due to on_rq, having incompletely
updated the list.  In this case the second for_each_sched_entity loop can
further modify se. The later code to fix up the list management fails to do
what is needed because se does not point to the sched_entity which broke out
of the first loop. The list is not fixed up because the throttled parent was
already added back to the list by a task enqueue in a parallel child hierarchy.

Address this by calling list_add_leaf_cfs_rq if there are throttled parents
while doing the second for_each_sched_entity loop.

v3: clean up commit message and add fixes and review tags.

Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>
Signed-off-by: Phil Auld <pauld@...hat.com>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Fixes: fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
---
 kernel/sched/fair.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 02f323b85b6d..c6d57c334d51 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5479,6 +5479,13 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 		/* end evaluation on encountering a throttled cfs_rq */
 		if (cfs_rq_throttled(cfs_rq))
 			goto enqueue_throttle;
+
+               /*
+                * One parent has been throttled and cfs_rq removed from the
+                * list. Add it back to not break the leaf list.
+                */
+               if (throttled_hierarchy(cfs_rq))
+                       list_add_leaf_cfs_rq(cfs_rq);
 	}

 enqueue_throttle:
-- 
2.18.0

--