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

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Message-ID: <20160617162837.GQ5981@e106622-lin>
Date:	Fri, 17 Jun 2016 17:28:37 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	mingo@...hat.com, luca.abeni@...tn.it
Subject: Re: [PATCH] sched/deadline: remove useless param from
 setup_new_dl_entity

On 17/06/16 09:49, Steven Rostedt wrote:
> On Fri, 17 Jun 2016 10:48:41 +0100
> Juri Lelli <juri.lelli@....com> wrote:
> 
> > setup_new_dl_entity() takes two parameters, but it only actually uses
> > one of them to setup a new dl_entity.
> > 
> 
> Actually this patch is making it so that setup_new_dl_entity() only
> uses one of the parameters. Can you note why that change happened.
> Because this change log implies that the second parameter wasn't used
> before this patch, and that is incorrect.
> 

True, but we were practically already using the same parameter, under a
different name though, after

2f9f3fdc928 "sched/deadline: Remove dl_new from struct sched_dl_entity"

as we currently do:

  setup_new_dl_entity(&p->dl, &p->dl)

> This patch reverts part of the change done in
> commit 2d3d891d334 "sched/deadline: Add SCHED_DEADLINE inheritance
> logic"
> 

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. So, I guess the question
is actually why we wanted to use pi_se's parameters (the potential PI
donor) for setting up a new entity? Maybe we broke the situation where a
task is currently boosted by a DEADLINE waiter and we swich the holder
to DEADLINE?

> It would be nice to have the reason in the change log.
> 

Thanks a lot for pointing out what might be more than inaccuracy in the
changelog.

Best,

- Juri

> 
> > Remove the second, useless, parameter.
> > 
> > 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>
> > ---
> >  kernel/sched/deadline.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index fcb7f0217ff4..5229788a4765 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -346,8 +346,7 @@ 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);
> > @@ -367,8 +366,8 @@ static inline void setup_new_dl_entity(struct sched_dl_entity *dl_se,
> >  	 * future; in fact, we must consider execution overheads (time
> >  	 * spent on hardirq context, etc.).
> >  	 */
> > -	dl_se->deadline = rq_clock(rq) + pi_se->dl_deadline;
> > -	dl_se->runtime = pi_se->dl_runtime;
> > +	dl_se->deadline = rq_clock(rq) + dl_se->dl_deadline;
> > +	dl_se->runtime = dl_se->dl_runtime;
> >  }
> >  
> >  /*
> > @@ -1721,7 +1720,7 @@ 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);
> >  
> >  	if (task_on_rq_queued(p) && rq->curr != p) {
> >  #ifdef CONFIG_SMP
>