linux-kernel - Re: [PATCH] sched/deadline: Remove unnecessary condition in push_dl

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Date:   Wed, 15 Feb 2017 12:37:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@....com>
Cc:     Byungchul Park <byungchul.park@....com>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        kernel-team@....com, Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>
Subject: Re: [PATCH] sched/deadline: Remove unnecessary condition in
 push_dl_task()

On Wed, Feb 15, 2017 at 10:47:49AM +0000, Juri Lelli wrote:
> [+Steve, Luca]
> 
> Hi,
> 
> On 15/02/17 14:11, Byungchul Park wrote:
> > Once pick_next_pushable_dl_task(rq) return a task, it guarantees that
> > the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> > task == next_task. Remove a redundant condition and make code simpler.
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > ---
> >  kernel/sched/deadline.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 27737f3..ad8d577 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -1483,7 +1483,7 @@ static int push_dl_task(struct rq *rq)
> >  		 * then possible that next_task has migrated.
> >  		 */
> >  		task = pick_next_pushable_dl_task(rq);
> > -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> > +		if (task == next_task) {
> 
> Seems a sensible optimization to me. Actually, we are doing the same for
> rt.c; Steve, Peter, do you think we should optimize that as well?
> 

If correct (and I've not spend brain cycles on that) yes. We should keep
this push-pull muck synced between rt and deadline as much as possible.