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

[Juri Lelli]

Hi,

On 30/12/16 12:33, Luca Abeni wrote:
> From: Luca Abeni <luca.abeni@xxxxxxxx>
> 
> 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@xxxxxxxx>
> ---

[...]

> +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.

> +			 */
> +			if (hrtimer_try_to_cancel(&dl_se->inactive_timer) == 1)
> +				put_task_struct(dl_task_of(dl_se));
> +			WARN_ON(dl_task_of(dl_se)->nr_cpus_allowed > 1);
> +			dl_se->dl_non_contending = 0;
> +		} else {

[...]

> @@ -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?

Best,

- Juri