linux-kernel - Re: [PATCH V3 1/3] sched/deadline: Replenishment timer should fire in the next period

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Date:   Tue, 7 Mar 2017 14:21:04 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mike Galbraith <efault@....de>,
        Romulo Silva de Oliveira <romulo.deoliveira@...c.br>
Subject: Re: [PATCH V3 1/3] sched/deadline: Replenishment timer should fire in
 the next period

2017-02-28 18:07 GMT+08:00 Daniel Bristot de Oliveira <bristot@...hat.com>:
> Currently, the replenishment timer is set to fire at the deadline
> of a task. Although that works for implicit deadline tasks because the
> deadline is equals to the begin of the next period, that is not correct
> for constrained deadline tasks (deadline < period).
>
> For instance:
>
> f.c:
>  --------------- %< ---------------
> int main (void)
> {
>         for(;;);
> }
>  --------------- >% ---------------
>
>   # gcc -o f f.c
>
>   # trace-cmd record -e sched:sched_switch                              \
>                                    -e syscalls:sys_exit_sched_setattr   \
>    chrt -d --sched-runtime  490000000                                   \
>            --sched-deadline 500000000                                   \
>            --sched-period  1000000000 0 ./f
>
>   # trace-cmd report | grep "{pid of ./f}"
>
> After setting parameters, the task is replenished and continue running
> until being throttled.
>          f-11295 [003] 13322.113776: sys_exit_sched_setattr: 0x0
>
> The task is throttled after running 492318 ms, as expected.

This should be us.

Otherwise,

Reviewed-by: Wanpeng Li <wanpeng.li@...mail.com>

>          f-11295 [003] 13322.606094: sched_switch:   f:11295 [-1] R ==> \
>                                                    watchdog/3:32 [0]
>
> But then, the task is replenished 500719 ms after the first
> replenishment:
>     <idle>-0     [003] 13322.614495: sched_switch:   swapper/3:0 [120] R \
>                                                    ==> f:11295 [-1]
>
> Running for 490277 ms:
>          f-11295 [003] 13323.104772: sched_switch:   f:11295 [-1] R ==>  \
>                                                    swapper/3:0 [120]
>
> Hence, in the first period, the task runs 2 * runtime, and that is a bug.
>
> During the first replenishment, the next deadline is set one period away.
> So the runtime / period starts to be respected. However, as the second
> replenishment took place in the wrong instant, the next replenishment
> will also be held in a wrong instant of time. Rather than occurring in
> the nth period away from the first activation, it is taking place
> in the (nth period - relative deadline).
>
> Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
> Reviewed-by: Luca Abeni <luca.abeni@...tannapisa.it>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> Reviewed-by: Juri Lelli <juri.lelli@....com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@....com>
> Cc: Tommaso Cucinotta <tommaso.cucinotta@...up.it>
> Cc: Luca Abeni <luca.abeni@...tannapisa.it>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Mike Galbraith <efault@....de>
> Cc: Romulo Silva de Oliveira <romulo.deoliveira@...c.br>
> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> Cc: linux-kernel@...r.kernel.org
> ---
>  kernel/sched/deadline.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 70ef2b1..3c94d85 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -505,10 +505,15 @@ static void update_dl_entity(struct sched_dl_entity *dl_se,
>         }
>  }
>
> +static inline u64 dl_next_period(struct sched_dl_entity *dl_se)
> +{
> +       return dl_se->deadline - dl_se->dl_deadline + dl_se->dl_period;
> +}
> +
>  /*
>   * If the entity depleted all its runtime, and if we want it to sleep
>   * while waiting for some new execution time to become available, we
> - * set the bandwidth enforcement timer to the replenishment instant
> + * set the bandwidth replenishment timer to the replenishment instant
>   * and try to activate it.
>   *
>   * Notice that it is important for the caller to know if the timer
> @@ -530,7 +535,7 @@ static int start_dl_timer(struct task_struct *p)
>          * that it is actually coming from rq->clock and not from
>          * hrtimer's time base reading.
>          */
> -       act = ns_to_ktime(dl_se->deadline);
> +       act = ns_to_ktime(dl_next_period(dl_se));
>         now = hrtimer_cb_get_time(timer);
>         delta = ktime_to_ns(now) - rq_clock(rq);
>         act = ktime_add_ns(act, delta);
> --
> 2.9.3
>