linux-kernel - Re: [PATCH v3] sched/deadline: Change the time to replenish runtime for sleep tasks

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Date:   Tue, 28 Feb 2017 17:47:53 +0000
From:   Juri Lelli <juri.lelli@....com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     peterz@...radead.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        rostedt@...dmis.org, kernel-team@....com
Subject: Re: [PATCH v3] sched/deadline: Change the time to replenish runtime
 for sleep tasks

On 28/02/17 22:09, Byungchul Park wrote:
> On Tue, Feb 28, 2017 at 11:35:15AM +0000, Juri Lelli wrote:
> > Hi,
> > 
> > On 23/02/17 15:14, Byungchul Park wrote:
> > > Let's consider the following example.
> > > 
> > > timeline : o...................o.........o.......o..o
> > >            ^                   ^         ^       ^  ^
> > >            |                   |         |       |  |
> > >        start                   |         |       |  |
> > >                 original runtime         |       |  |
> > >                      sleep with (-)runtime       |  |
> > >                                  original deadline  |
> > >                                               wake up
> > > 
> > > When this task is woken up, a negative runtime should be considered,
> > > which means that the task should get penalized when assigning runtime,
> > > becasue it already spent more than expected. Current code handles this
> > > by replenishing a runtime in hrtimer callback for deadline. But this
> > > approach has room for improvement:
> > > 
> > >    It will be replenished twice unnecessarily if the task sleeps for
> > >    long time so that the deadline, assigned in the hrtimer callback,
> > >    also passed. In other words, one happens in the callback and the
> > >    other happens in update_dl_entiry() when waking it up.
> > > 
> > > So force to replenish it for sleep tasks when waking it up.
> > > 
> > > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > > ---
> > >  kernel/sched/deadline.c | 13 ++++++-------
> > >  1 file changed, 6 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index 27737f3..cb43ce9 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -498,8 +498,9 @@ static void update_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);
> > >  
> > > -	if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
> > > -	    dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
> > > +	if (dl_time_before(dl_se->deadline, rq_clock(rq)))
> > > +		replenish_dl_entity(dl_se, pi_se);
> > > +	else if (dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
> > >  		dl_se->deadline = rq_clock(rq) + pi_se->dl_deadline;
> > >  		dl_se->runtime = pi_se->dl_runtime;
> > >  	}
> > > @@ -621,13 +622,11 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer)
> > >  	 *         __dequeue_task_dl()
> > >  	 *     prev->on_rq = 0;
> > >  	 *
> > > -	 * We can be both throttled and !queued. Replenish the counter
> > > -	 * but do not enqueue -- wait for our wakeup to do that.
> > > +	 * We can be both throttled and !queued. Wait for our wakeup to
> > > +	 * replenish runtime and enqueue p.
> > >  	 */
> > > -	if (!task_on_rq_queued(p)) {
> > > -		replenish_dl_entity(dl_se, dl_se);
> > 
> > Hasn't this patch the same problem we discussed a couple of weeks ago?
> 
> No. This patch solves the problem by calling replenish_dl_entity() when
> a dl task is woken up.
> 

So, if the task was throttled in the "going to sleep" path we set the
replenishment timer to fire at your "original deadline" instant of time
above. Now, 3 things can happen I guess:

 - task wakes up before the replenishment timer ("original deadline")
   -> it is throttled, so we do nothing
 
 - task wakes up after the replenishment timer
   -> we replenished it in the timer callback (which considers negative
      runtime from previous execution)
      + deadline should be in the future
      + dl_entity shouldn't overflow
      + we don't touch its parameters, but we simply enqueue it back on dl_rq

 - task wakes up even after the deadline it has got from previous
   replenishment expired
   -> we assign to him completely new parameters, but since he didn't
      use the previous runtime at all, this should be fine I guess

What am I still missing? :)

> The problem was that it cannot consider negative runtime if we replenish
> the task when it's woken up. So I made replenish_dl_entity() called even
> on wake-up path, instead of simple assignment.
> 
> IMHO, this patch avoids double-replenishing properly, but adds additional
> condition on wake-up path to acheive it. To be honest, I don't think it's
> worth as I expected.
> 
> Thank you,
> Byungchul
> 
> > 
> > https://marc.info/?l=linux-kernel&m=148699950802995
> > 
> > Thanks,
> > 
> > - Juri