linux-kernel - Re: [PATCH v2] sched/fair: Fix enqueue_task

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Date:   Tue, 12 May 2020 16:06:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Phil Auld <pauld@...hat.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v2] sched/fair: Fix enqueue_task_fair warning some more

On Mon, May 11, 2020 at 04:44:10PM -0400, Phil Auld wrote:
> Peter/Ingo, do you want me to resend or can you fix when applying?

I now have this, do I need more edits?

---

Subject: sched/fair: Fix enqueue_task_fair warning some more
From: Phil Auld <pauld@...hat.com>
Date: Thu, 7 May 2020 16:36:12 -0400

From: Phil Auld <pauld@...hat.com>

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 no longer points to the sched_entity which broke
out of the first loop.

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

Fixes: fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>
Signed-off-by: Phil Auld <pauld@...hat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>
Link: https://lkml.kernel.org/r/20200507203612.GF19331@lorien.usersys.redhat.com
---
 kernel/sched/fair.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5479,6 +5479,13 @@ enqueue_task_fair(struct rq *rq, struct
 		/* 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: