linux-kernel - Re: [PATCH v2] sched/pelt: fix false running accounting

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Date:   Tue, 4 Jul 2017 11:12:34 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2] sched/pelt: fix false running accounting

On 4 July 2017 at 10:34, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jul 04, 2017 at 09:27:07AM +0200, Peter Zijlstra wrote:
>> On Sat, Jul 01, 2017 at 07:06:13AM +0200, Vincent Guittot wrote:
>> > The running state is a subset of runnable state which means that running
>> > can't be set if runnable (weight) is cleared. There are corner cases
>> > where the current sched_entity has been already dequeued but cfs_rq->curr
>> > has not been updated yet and still points to the dequeued sched_entity.
>> > If ___update_load_avg is called at that time, weight will be 0 and running
>> > will be set which is not possible.
>> >
>> > This case happens during pick_next_task_fair() when a cfs_rq becomes idles.
>> > The current sched_entity has been dequeued so se->on_rq is cleared and
>> > cfs_rq->weight is null. But cfs_rq->curr still points to se (it will be
>> > cleared when picking the idle thread). Because the cfs_rq becomes idle,
>> > idle_balance() is called and ends up to call update_blocked_averages()
>> > with these wrong running and runnable states.
>> >
>> > Add a test in ___update_load_avg to correct the running state in this case.
>>
>> Cute, however did you find that ?
>
> Hmm,.. could you give a little more detail?
>
> Because if ->on_rq=0, we'll have done dequeue_task() which will have
> done update_curr() with ->on_rq, weight and ->running consistently.
>
> Then the above, inconsistent update should not happen, because delta=0.

In fact, the delta between dequeue_entity_load_avg() and
update_blocked_averages() is not 0 on my platform (hikey) but can be
longer than 60us (at lowest frequency with only 1 task group level)