linux-kernel - Re: [PATCH] sched/deadline: Remove redundant code replenishing runtime

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Date:   Mon, 13 Feb 2017 13:29:41 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Juri Lelli <juri.lelli@....com>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, tkhai@...dex.ru
Subject: Re: [PATCH] sched/deadline: Remove redundant code replenishing
 runtime

On Mon, Feb 13, 2017 at 11:30:09AM +0900, Byungchul Park wrote:
> On Fri, Feb 10, 2017 at 01:39:33PM +0000, Juri Lelli wrote:
> > Hi,
> > 
> > On 10/02/17 18:11, Byungchul Park wrote:
> > > For a task passing its deadline while !rq, it will be replenished
> > > in the following path because dl_se->deadline < rq_lock.
> > > 
> > >    enqueue_dl_entity(ENQUEUE_WAKEUP)
> > >       update_dl_entity
> > > 
> > > Therefore, code replenishing it in the timer callback in the case is
> > > unnecessary. This is not for enhancing performance but just for removing
> > > a redundant code.
> > > 
> > > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > > ---
> > >  kernel/sched/deadline.c | 4 +---
> > >  1 file changed, 1 insertion(+), 3 deletions(-)
> > > 
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index 27737f3..9c77696 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -624,10 +624,8 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
> > >  	 * We can be both throttled and !queued. Replenish the counter
> > >  	 * but do not enqueue -- wait for our wakeup to do that.
> > >  	 */
> > > -	if (!task_on_rq_queued(p)) {
> > > -		replenish_dl_entity(dl_se, dl_se);
> > 
> > I think we actually want to replenish and set the next deadline at this
> > point of time, not the one that we get when the task will eventually wake up.
> 
> Hello juri,
> 
> But I wonder if it's meaningful to set a next deadline for a 'sleeping
> task', which, rather, could be worse because its bandwidth might be
> distorted at the time it's woken up.
> 
> IMHO, it's neat to set its deadline and runtime when being woken up, in
> the case already passed its deadline. Am I wrong?

And I found that dl_entity_overflow() returns true and replenishes the
task unconditionally in update_dl_entity() again when the task is woken
up, because 'runtime / (deadline - t) > dl_runtime / dl_period' is true.

In other words, replenishing the sleeping task in timer callback is
totally unnecessary and redundant work.

> 
> Thank you,
> Byungchul