linux-kernel - Re: [PATCH v4] sched/deadline: remove useless param from setup_new_dl

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Date:	Fri, 15 Jul 2016 18:39:35 +0800
From:	Xunlei Pang <xpang@...hat.com>
To:	Juri Lelli <juri.lelli@....com>, peterz@...radead.org
Cc:	rostedt@...dmis.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com, luca.abeni@...tn.it
Subject: Re: [PATCH v4] sched/deadline: remove useless param from
 setup_new_dl_entity

On 2016/07/13 at 18:58, Juri Lelli wrote:
> setup_new_dl_entity() takes two parameters, but it only actually uses
> one of them, under a different name, to setup a new dl_entity, after:
>
>  2f9f3fdc928 "sched/deadline: Remove dl_new from struct sched_dl_entity"
>
> as we currently do
>
>  setup_new_dl_entity(&p->dl, &p->dl)
>
> However, before Luca's change we were doing
>
>  setup_new_dl_entity(dl_se, pi_se)
>
> in update_dl_entity() for a dl_se->new entity: we were using pi_se's
> parameters (the potential PI donor) for setting up a new entity.
>
> Restore this behaviour (as we want to correctly initialize parameters of
> a boosted task that enters DEADLINE) by removing the useless second
> parameter of setup_new_dl_entity() and retrieving the top waiter
> directly from inside that function.
>
> While we are at it we also optimize things further calling setup_new_dl_
> entity only for already queued tasks, since (as pointed out by Xunlei)
> we already do the very same update at tasks wakeup time anyway.
>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Luca Abeni <luca.abeni@...tn.it>
> Cc: Xunlei Pang <xpang@...hat.com>
> Signed-off-by: Juri Lelli <juri.lelli@....com>
>
> ---
>  Changes from v3:
>    - call setup_new_dl_entity only if task is enqueued already
>
>  Changes from v2:
>    - optimize by calling rt_mutex_get_top_task only if the task is
>      boosted (as suggested by Steve)
>
>  Changes from v1:
>    - Steve pointed out that we were actually using the second parameter
>      to permorm initialization
>    - Luca confirmed that behavior is slightly changed w.r.t. before his
>      change
>    - changelog updated and original behavior restored
> ---
>  kernel/sched/deadline.c | 39 ++++++++++++++++++++++++++++-----------
>  1 file changed, 28 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index fcb7f0217ff4..dc56f5be0112 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -346,11 +346,12 @@ static void check_preempt_curr_dl(struct rq *rq, struct task_struct *p,
>   * one, and to (try to!) reconcile itself with its own scheduling
>   * parameters.
>   */
> -static inline void setup_new_dl_entity(struct sched_dl_entity *dl_se,
> -				       struct sched_dl_entity *pi_se)
> +static inline void setup_new_dl_entity(struct sched_dl_entity *dl_se)
>  {
>  	struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
>  	struct rq *rq = rq_of_dl_rq(dl_rq);
> +	struct task_struct *pi_task;
> +	struct sched_dl_entity *pi_se = dl_se;
>  
>  	WARN_ON(dl_time_before(rq_clock(rq), dl_se->deadline));
>  
> @@ -363,6 +364,15 @@ static inline void setup_new_dl_entity(struct sched_dl_entity *dl_se,
>  		return;
>  
>  	/*
> +	 * Use the scheduling parameters of the top pi-waiter task,
> +	 * if we have one from which we can inherit a deadline.
> +	 */
> +	if (dl_se->dl_boosted &&
> +	    (pi_task = rt_mutex_get_top_task(dl_task_of(dl_se))) &&
> +	    dl_prio(pi_task->normal_prio))
> +		pi_se = &pi_task->dl;

Since this is only called for queued cases now, there is no need to
check boosted stuff here. As enqueue_task(ENQUEUE_REPLENISH)
is called before check_class_changed() in rt_mutex_setprio().

Regards,
Xunlei

> +
> +	/*
>  	 * We use the regular wall clock time to set deadlines in the
>  	 * future; in fact, we must consider execution overheads (time
>  	 * spent on hardirq context, etc.).
> @@ -1720,19 +1730,26 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
>   */
>  static void switched_to_dl(struct rq *rq, struct task_struct *p)
>  {
> -	if (dl_time_before(p->dl.deadline, rq_clock(rq)))
> -		setup_new_dl_entity(&p->dl, &p->dl);
>  
> -	if (task_on_rq_queued(p) && rq->curr != p) {
> +	if (task_on_rq_queued(p)) {
> +		/*
> +		 * If p is not queued we will update its parameters at next
> +		 * wakeup.
> +		 */
> +		if (dl_time_before(p->dl.deadline, rq_clock(rq)))
> +			setup_new_dl_entity(&p->dl);
> +
> +		if (rq->curr != p) {
>  #ifdef CONFIG_SMP
> -		if (tsk_nr_cpus_allowed(p) > 1 && rq->dl.overloaded)
> -			queue_push_tasks(rq);
> +			if (tsk_nr_cpus_allowed(p) > 1 && rq->dl.overloaded)
> +				queue_push_tasks(rq);
>  #else
> -		if (dl_task(rq->curr))
> -			check_preempt_curr_dl(rq, p, 0);
> -		else
> -			resched_curr(rq);
> +			if (dl_task(rq->curr))
> +				check_preempt_curr_dl(rq, p, 0);
> +			else
> +				resched_curr(rq);
>  #endif
> +		}
>  	}
>  }
>