Re: [RFC v4 2/6] sched/deadline: improve the tracking of active utilization

[luca abeni <luca.abeni@santannapisa.it>]

On Wed, 11 Jan 2017 17:05:42 +0000
Juri Lelli <juri.lelli@arm.com> wrote:

> Hi,
> 
> On 30/12/16 12:33, Luca Abeni wrote:
> > From: Luca Abeni <luca.abeni@unitn.it>
> > 
> > This patch implements a more theoretically sound algorithm for
> > tracking active utilization: instead of decreasing it when a
> > task blocks, use a timer (the "inactive timer", named after the
> > "Inactive" task state of the GRUB algorithm) to decrease the
> > active utilization at the so called "0-lag time".
> > 
> > Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
> > ---  
> 
> [...]
> 
> > +static enum hrtimer_restart inactive_task_timer(struct hrtimer
> > *timer) +{
> > +	struct sched_dl_entity *dl_se = container_of(timer,
> > +						     struct
> > sched_dl_entity,
> > +
> > inactive_timer);
> > +	struct task_struct *p = dl_task_of(dl_se);
> > +	struct rq_flags rf;
> > +	struct rq *rq;
> > +
> > +	rq = task_rq_lock(p, &rf);
> > +
> > +	if (!dl_task(p) || p->state == TASK_DEAD) {
> > +		if (p->state == TASK_DEAD &&
> > dl_se->dl_non_contending)
> > +			sub_running_bw(&p->dl,
> > dl_rq_of_se(&p->dl)); +
> > +		__dl_clear_params(p);
> > +
> > +		goto unlock;
> > +	}
> > +	if (dl_se->dl_non_contending == 0)
> > +		goto unlock;
> > +
> > +	sched_clock_tick();
> > +	update_rq_clock(rq);
> > +
> > +	sub_running_bw(dl_se, &rq->dl);
> > +	dl_se->dl_non_contending = 0;
> > +unlock:
> > +	task_rq_unlock(rq, p, &rf);
> > +	put_task_struct(p);
> > +
> > +	return HRTIMER_NORESTART;
> > +}
> > +  
> 
> [...]
> 
> >  static void inc_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
> > @@ -934,7 +1014,28 @@ enqueue_dl_entity(struct sched_dl_entity
> > *dl_se, if (flags & ENQUEUE_WAKEUP) {
> >  		struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
> >  
> > -		add_running_bw(dl_se, dl_rq);
> > +		if (dl_se->dl_non_contending) {
> > +			/*
> > +			 * If the timer handler is currently
> > running and the
> > +			 * timer cannot be cancelled,
> > inactive_task_timer()
> > +			 * will see that dl_not_contending is not
> > set, and
> > +			 * will do nothing, so we are still safe.  
> 
> Here and below: the timer callback will actually put_task_struct()
> (see above) if dl_not_contending is not set; that's why we don't need
> to do that if try_to_cancel returned -1 (or 0). Saying "will do
> nothing" is a bit misleading, IMHO.

Sorry... I originally had a bug with this put_task_struct() thing. The
bug is now (hopefully :) fixed, but I forgot to update the comment...
I'll fix it for next submission.

> > @@ -1097,6 +1198,22 @@ select_task_rq_dl(struct task_struct *p, int
> > cpu, int sd_flag, int flags) }
> >  	rcu_read_unlock();
> >  
> > +	rq = task_rq(p);
> > +	raw_spin_lock(&rq->lock);
> > +	if (p->dl.dl_non_contending) {
> > +		sub_running_bw(&p->dl, &rq->dl);
> > +		p->dl.dl_non_contending = 0;
> > +		/*
> > +		 * If the timer handler is currently running and
> > the
> > +		 * timer cannot be cancelled, inactive_task_timer()
> > +		 * will see that dl_not_contending is not set, and
> > +		 * will do nothing, so we are still safe.
> > +		 */
> > +		if (hrtimer_try_to_cancel(&p->dl.inactive_timer)
> > == 1)
> > +			put_task_struct(p);
> > +	}
> > +	raw_spin_unlock(&rq->lock);
> > +
> >  out:
> >  	return cpu;
> >  }  
> 
> We already raised the issue about having to lock the rq in
> select_task_rq_dl() while reviewing the previous version; did you have
> any thinking about possible solutions? Maybe simply bail out (need to
> see how frequent this is however) or use an inner lock?

Sorry; I did not come up with any good idea for avoiding to lock the
rq... I'll think about this again... The only alternative idea I have
is just to avoid changing cpu, but I do not know if it is acceptable...




			Thanks,
				Luca