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

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Date:   Tue, 12 May 2020 11:00:16 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Phil Auld <pauld@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.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 11/05/2020 22:44, Phil Auld wrote:
> On Mon, May 11, 2020 at 09:25:43PM +0200 Vincent Guittot wrote:
>> On Thu, 7 May 2020 at 22:36, Phil Auld <pauld@...hat.com> wrote:
>>>
>>> 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 leaf_add_rq_list 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>
>>> 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>
>>
>> With the Fixes tag and the typo mentioned by Tao
>>
> 
> Right, that last line of the commit message should read "list_add_leaf_cfs_rq"
> 
> 
>> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> 
> Thanks Vincent.
> 
> Peter/Ingo, do you want me to resend or can you fix when applying?


Maybe you could add that 'the throttled parent was already added back to
the list by a task enqueue in a parallel child hierarchy'.

IMHO, this is part of the description because otherwise the throttled
parent would have connected the branch.

And the not-adding of the intermediate child cfs_rq would have gone
unnoticed.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

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