linux-kernel - Re: [PATCH v3] sched/deadline: remove useless param from setup_new_dl

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Date:	Mon, 11 Jul 2016 13:03:56 +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 v3] sched/deadline: remove useless param from
 setup_new_dl_entity

On 2016/07/08 at 19:28, 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.
>
> 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>
> Signed-off-by: Juri Lelli <juri.lelli@....com>
>
> ---
>  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 | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index fcb7f0217ff4..7aeb21963d3c 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;
> +
> +	/*
>  	 * 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.).
> @@ -1721,7 +1731,7 @@ 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);
> +		setup_new_dl_entity(&p->dl);

I'm curious why we even call setup_new_dl_entity() for non-queued cases?
It seems more reasonable to do it when it really gets queued. We can see
that enqueue_task_dl()->update_dl_entity() also has the same update logic
as switched_to_dl().

If so, for already queued and boosted cases, rt_mutex_setprio() will call
enqueue_task() with ENQUEUE_REPLENISH set, so enqueue_dl_entity()
->replenish_dl_entity() will advance p->dl.deadline beforehand, see code:
replenish_dl_entity():
    if (dl_se->dl_deadline == 0) {
        dl_se->deadline = rq_clock(rq) + pi_se->dl_deadline;
        dl_se->runtime = pi_se->dl_runtime;
    }

IOW, we don't need to handle !dl boosted cases in setup_new_dl_entity().

Regards,
Xunlei

>  
>  	if (task_on_rq_queued(p) && rq->curr != p) {
>  #ifdef CONFIG_SMP