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

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Date:   Sat, 1 Jul 2017 07:09:43 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/pelt: fix false running accounting in PELT

On 30 June 2017 at 15:58, Vincent Guittot <vincent.guittot@...aro.org> 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.*
>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>

The v1 is just wrong. I have sent a v2 with correct patch

sorry for the disturb
Vincent

> ---
>  kernel/sched/fair.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 008c514..5fdcb42 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2968,6 +2968,24 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
>         sa->last_update_time += delta << 10;
>
>         /*
> +        * running is a subset of runnable (weight) so running can't be set if
> +        * runnable is clear. But there are some corner cases where the current
> +        * se has been already dequeued but cfs_rq->curr still points to it.
> +        * This means that weight will be 0 but not running for a sched_entity
> +        * but also for a cfs_rq if the latter becomes idle. As an example,
> +        * this happens during idle_balance() which calls
> +        * update_blocked_averages()
> +        */
> +       if (weight)
> +               running = 1;
> +
> +       /*
> +        * Scale time to reflect the amount a computation effectively done
> +        * during the time slot at current capacity
> +        */
> +       delta = scale_time(delta, cpu, sa, weight, running);
> +
> +       /*
>          * Now we know we crossed measurement unit boundaries. The *_avg
>          * accrues by two steps:
>          *
> --
> 2.7.4
>