linux-kernel - Re: [PATCH v4 2/5] sched/fair: Fix attaching task sched avgs twice when switching to fair or changing task group

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Message-ID: <CAKfTPtD5e1DnCCTTQ54Df8ybA6YKR_5=d=tsoNraZSiAs6JbOA@mail.gmail.com>
Date:	Tue, 7 Jun 2016 10:09:14 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v4 2/5] sched/fair: Fix attaching task sched avgs twice
 when switching to fair or changing task group

On 6 June 2016 at 21:05, Yuyang Du <yuyang.du@...el.com> wrote:
> Hi Vincent,
>
> On Mon, Jun 06, 2016 at 02:32:39PM +0200, Vincent Guittot wrote:
>> > Unlike the switch to fair class case, if the task is on_rq, it will be
>> > enqueued after we move task groups, so the simplest solution is to reset
>> > the task's last_update_time when we do task_move_group(), but not to
>> > attach sched avgs in task_move_group(), and then let enqueue_task() do
>> > the sched avgs attachment.
>>
>> According to the review of the previous version
>> http://www.gossamer-threads.com/lists/linux/kernel/2450678#2450678,
>> only the use case switch to fair with a task that has never been
>> queued as a CFS task before, can have the issue and this will not
>> happen for other use cases described above.
>> So can you limit the description in the commit message to just this
>> use case unless you have discovered new use cases and in this case
>> please add the description.
>
> I assure you that I will, :)
>
>> Then, the problem with this use case, comes that last_update_time == 0
>> has a special meaning ( task has migrated ) and we initialize
>> last_update_time with this value.
>
> In general, the meaning of last_update_time == 0 is: not yet attached to
> any cfs queue.
>
>> A much more simple solution would be
>> to prevent last_update_time to be initialized with this special value.
>> We can initialize the last_update_time of a sched_entity to 1 as an
>> example which is easier than these changes.
>
> Then at least, we must differentiate CONFIG_FAIR_GROUP_SCHED and
> !CONFIG_FAIR_GROUP_SCHED. And I am not sure whether this is a simpler

Why do you want to differentiate ? we already have
sa->last_update_time = 0; in init_entity_runnable_average. We just
have to change it by sa->last_update_time = 1;

> solution, as I haven't sorted out the whole thing. IMO, this solution
> seems a little too hacky and I'd prefer not if we have alternative.