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

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Date:	Mon, 11 Jul 2016 10:01:51 +0200
From:	luca abeni <luca.abeni@...tn.it>
To:	Xunlei Pang <xpang@...hat.com>
Cc:	Juri Lelli <juri.lelli@....com>, peterz@...radead.org,
	rostedt@...dmis.org, linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: [PATCH v3] sched/deadline: remove useless param from
 setup_new_dl_entity

Hello,

On Mon, 11 Jul 2016 13:03:56 +0800
Xunlei Pang <xpang@...hat.com> wrote:

> On 2016/07/08 at 19:28, Juri Lelli wrote:
[...]
> > @@ -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().

I wondered the same when removing the dl_new field from
sched_dl_entity... But then I realised that enqueue_dl_entity() does
not always invoke update_dl_entity() or replenish_dl_entity()... For
example, when a task switches from SCHED_OTHER (or RT) to -deadline due
to sched_setattr() (or similar) these functions are not invoked.


				Luca

> 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  
>