linux-kernel - Re: [PATCH V3 2/3] sched/deadline: Throttle a constrained deadline task activated after the deadline

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Date:   Tue, 28 Feb 2017 11:04:53 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     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>,
        Mike Galbraith <efault@....de>,
        Romulo Silva de Oliveira <romulo.deoliveira@...c.br>
Subject: Re: [PATCH V3 2/3] sched/deadline: Throttle a constrained deadline
 task activated after the deadline

On Tue, 28 Feb 2017 11:07:54 +0100
Daniel Bristot de Oliveira <bristot@...hat.com> wrote:


> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 3c94d85..3a88def 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -694,6 +694,37 @@ void init_dl_task_timer(struct sched_dl_entity *dl_se)
>  	timer->function = dl_task_timer;
>  }
>  
> +/*
> + * During the activation, CBS checks if it can reuse the current task's
> + * runtime and period. If the deadline of the task is in the past, CBS
> + * cannot use the runtime, and so it replenishes the task. This rule
> + * works fine for implicit deadline tasks (deadline == period), and the
> + * CBS was designed for implicit deadline tasks. However, a task with
> + * constrained deadline (deadine < period) might be awakened after the
> + * deadline, but before the next period. In this case, replenishing the
> + * task would allow it to run for runtime / deadline. As in this case
> + * deadline < period, CBS enables a task to run for more than the
> + * runtime / period. In a very load system, this can cause the domino

                        In a very loaded system, this can cause a domino

> + * effect, making other tasks to miss their deadlines.

      effect, making other tasks miss their deadlines.

> + *
> + * To avoid this problem, in the activation of a constrained deadline
> + * task after the deadline but before the next period, throttle the
> + * task and set the replenishing timer to the begin of the next period,

                                          to the beginning of the next

> + * unless it is boosted.
> + */
> +static inline void dl_check_constrained_dl(struct sched_dl_entity *dl_se)
> +{
> +	struct task_struct *p = dl_task_of(dl_se);
> +	struct rq *rq = rq_of_dl_rq(dl_rq_of_se(dl_se));
> +
> +	if (dl_time_before(dl_se->deadline, rq_clock(rq)) &&
> +	    dl_time_before(rq_clock(rq), dl_next_period(dl_se))) {
> +		if (unlikely(dl_se->dl_boosted || !start_dl_timer(p)))
> +			return;
> +		dl_se->dl_throttled = 1;
> +	}
> +}
> +
>  static
>  int dl_runtime_exceeded(struct sched_dl_entity *dl_se)
>  {
> @@ -927,6 +958,11 @@ static void dequeue_dl_entity(struct sched_dl_entity *dl_se)
>  	__dequeue_dl_entity(dl_se);
>  }
>  
> +static inline bool dl_is_constrained(struct sched_dl_entity *dl_se)
> +{
> +	return dl_se->dl_deadline < dl_se->dl_period;
> +}
> +
>  static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
>  {
>  	struct task_struct *pi_task = rt_mutex_get_top_task(p);
> @@ -953,6 +989,15 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
>  	}
>  
>  	/*
> +	 * Check if a constrained deadline task was activated
> +	 * after the deadline but before the next period.
> +	 * If that is the case, the task will be throttled and
> +	 * the replenishment timer will be set to the next period.
> +	 */
> +	if (!pi_se->dl_throttled && dl_is_constrained(pi_se))
> +		dl_check_constrained_dl(pi_se);
> +
> +	/*
>  	 * If p is throttled, we do nothing. In fact, if it exhausted
>  	 * its budget it needs a replenishment and, since it now is on
>  	 * its rq, the bandwidth timer callback (which clearly has not

Except for the grammar mistakes...

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve