linux-kernel - Re: [PATCH V2 2/2] sched/deadline: Use the revised wakeup rule for suspending constrained dl tasks

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Date:   Tue, 30 May 2017 13:31:30 +0200
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Xunlei Pang <xpang@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Romulo Silva de Oliveira <romulo.deoliveira@...c.br>
Subject: Re: [PATCH V2 2/2] sched/deadline: Use the revised wakeup rule for
 suspending constrained dl tasks

On 05/30/2017 01:29 PM, Peter Zijlstra wrote:
> On Mon, May 29, 2017 at 04:24:03PM +0200, Daniel Bristot de Oliveira wrote:
>> +/*
>> + * Regarding the deadline, a task with implicit deadline has a relative
>> + * deadline == relative period. A task with constrained deadline has a
>> + * relative deadline <= relative period.
>> + *
>> + * Linux supports constrained deadline tasks. However, there are some
>> + * restrictions applied only for tasks with constrained deadline which are
>> + * not implicit deadline tasks. See update_dl_entity() to know more about
>> + * such restrictions.
>> + *
>> + * The dl_is_constrained() returns true if the task has a constrained but
>> + * not implicit deadline.
>> + */
>> +static inline bool dl_is_constrained(struct sched_dl_entity *dl_se)
>> +{
>> +	return dl_se->dl_deadline < dl_se->dl_period;
>> +}
> 
> OK, given that:
> 
> implicit    - deadline == period
> constrained - deadline <= period
> 
> and thus constrained can still be implicit, our function
> dl_is_constrained() is wrong.
> 
> I've replaced the above with dl_is_implicit().
> 
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -740,19 +740,17 @@ update_dl_revised_wakeup(struct sched_dl
>  /*
>   * Regarding the deadline, a task with implicit deadline has a relative
>   * deadline == relative period. A task with constrained deadline has a
> - * relative deadline < relative period.
> + * relative deadline <= relative period.
>   *
> - * We supports constrained deadline tasks. However, there are some
> - * restrictions applied only for tasks with constrained deadline which are
> - * not implicit deadline tasks. See update_dl_entity() to know more about
> - * such restrictions.
> + * We support constrained deadline tasks. However, there are some restrictions
> + * applied only for tasks which do not have an implicit deadline. See
> + * update_dl_entity() to know more about such restrictions.
>   *
> - * The dl_is_constrained() returns true if the task has a constrained
> - * (not implicit) deadline.
> + * The dl_is_implicit() returns true if the task has an implicit deadline.
>   */
> -static inline bool dl_is_constrained(struct sched_dl_entity *dl_se)
> +static inline bool dl_is_implicit(struct sched_dl_entity *dl_se)
>  {
> -	return dl_se->dl_deadline < dl_se->dl_period;
> +	return dl_se->dl_deadline == dl_se->dl_period;
>  }
>  
>  /*
> @@ -794,7 +792,7 @@ static void update_dl_entity(struct sche
>  	if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
>  	    dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
>  
> -		if (unlikely(dl_is_constrained(dl_se) &&
> +		if (unlikely(!dl_is_implicit(dl_se) &&
>  			     !dl_time_before(dl_se->deadline, rq_clock(rq)) &&
>  			     !dl_se->dl_boosted)){
>  			update_dl_revised_wakeup(dl_se, rq);
> @@ -1386,7 +1384,7 @@ static void enqueue_task_dl(struct rq *r
>  	 * If that is the case, the task will be throttled and
>  	 * the replenishment timer will be set to the next period.
>  	 */
> -	if (!p->dl.dl_throttled && dl_is_constrained(&p->dl))
> +	if (!p->dl.dl_throttled && !dl_is_implicit(&p->dl))
>  		dl_check_constrained_dl(&p->dl);
>  
>  	if (p->on_rq == TASK_ON_RQ_MIGRATING || flags & ENQUEUE_RESTORE) {
> 

Acked-by: Daniel Bristot de Oliveira <bristot@...hat.com>

-- Daniel