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

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Message-ID: <CAKfTPtBRt5jJeEKUkFa78RdtrHGvBn8wh947rwYtxDnWyLrzsg@mail.gmail.com>
Date:   Tue, 4 Jul 2017 11:57:12 +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 11:44, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jul 04, 2017 at 11:12:34AM +0200, Vincent Guittot wrote:
>> 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)
>
> But but but, how can that happen? Should it not all be under the same
> rq->lock and thus have only a single update_rq_clock() and thus be at
> the same 'instant' ?

idle_balance() unlock rq->lock before calling  update_blocked_averages
And update_blocked_averages() starts by calling update_rq_clock()