Date:   Tue, 1 Nov 2016 16:45:43 +0000
From:   Juri Lelli <juri.lelli@....com>
To:     Luca Abeni <luca.abeni@...tn.it>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC v3 1/6] Track the active utilisation

Hi,

a few nitpicks on subject and changelog and a couple of questions below.

Subject should be changed to something like

 sched/deadline: track the active utilisation

On 24/10/16 16:06, Luca Abeni wrote:
> The active utilisation here is defined as the total utilisation of the

s/The active/Active/
s/here//
s/of the active/of active/

> active (TASK_RUNNING) tasks queued on a runqueue. Hence, it is increased
> when a task wakes up and is decreased when a task blocks.
> 
> When a task is migrated from CPUi to CPUj, immediately subtract the task's
> utilisation from CPUi and add it to CPUj. This mechanism is implemented by
> modifying the pull and push functions.
> Note: this is not fully correct from the theoretical point of view
> (the utilisation should be removed from CPUi only at the 0 lag time),

a more theoretically sound solution will follow.

> but doing the right thing would be _MUCH_ more complex (leaving the
> timer armed when the task is on a different CPU... Inactive timers should
> be moved from per-task timers to per-runqueue lists of timers! Bah...)

I'd remove this paragraph above.

> 
> The utilisation tracking mechanism implemented in this commit can be
> fixed / improved by decreasing the active utilisation at the so-called
> "0-lag time" instead of when the task blocks.

And maybe this as well, or put it as more information about the "more
theoretically sound" solution?

> 
> Signed-off-by: Juri Lelli <juri.lelli@....com>
> Signed-off-by: Luca Abeni <luca.abeni@...tn.it>
> ---
>  kernel/sched/deadline.c | 39 ++++++++++++++++++++++++++++++++++++++-
>  kernel/sched/sched.h    |  6 ++++++
>  2 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 37e2449..3d95c1d 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -43,6 +43,22 @@ static inline int on_dl_rq(struct sched_dl_entity *dl_se)
>  	return !RB_EMPTY_NODE(&dl_se->rb_node);
>  }
>  
> +static void add_running_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
> +{
> +	u64 se_bw = dl_se->dl_bw;
> +
> +	dl_rq->running_bw += se_bw;
> +}
> +
> +static void sub_running_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
> +{
> +	u64 se_bw = dl_se->dl_bw;
> +
> +	dl_rq->running_bw -= se_bw;
> +	if (WARN_ON(dl_rq->running_bw < 0))
> +		dl_rq->running_bw = 0;
> +}
> +
>  static inline int is_leftmost(struct task_struct *p, struct dl_rq *dl_rq)
>  {
>  	struct sched_dl_entity *dl_se = &p->dl;
> @@ -498,6 +514,8 @@ 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);
>  
> +	add_running_bw(dl_se, dl_rq);
> +
>  	if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
>  	    dl_entity_overflow(dl_se, pi_se, rq_clock(rq))) {
>  		dl_se->deadline = rq_clock(rq) + pi_se->dl_deadline;
> @@ -947,14 +965,19 @@ static void enqueue_task_dl(struct rq *rq, struct task_struct *p, int flags)
>  		return;
>  	}
>  
> +	if (p->on_rq == TASK_ON_RQ_MIGRATING)
> +		add_running_bw(&p->dl, &rq->dl);
> +
>  	/*
>  	 * If p is throttled, we do nothing. In fact, if it exhausted
>  	 * its budget it needs a replenishment and, since it now is on
>  	 * its rq, the bandwidth timer callback (which clearly has not
>  	 * run yet) will take care of this.
>  	 */
> -	if (p->dl.dl_throttled && !(flags & ENQUEUE_REPLENISH))
> +	if (p->dl.dl_throttled && !(flags & ENQUEUE_REPLENISH)) {
> +		add_running_bw(&p->dl, &rq->dl);

Don't rememeber if we discussed this already, but do we need to add the bw here
even if the task is not actually enqueued until after the replenishment timer
fires?

>  		return;
> +	}
>  
>  	enqueue_dl_entity(&p->dl, pi_se, flags);
>  
> @@ -972,6 +995,12 @@ static void dequeue_task_dl(struct rq *rq, struct task_struct *p, int flags)
>  {
>  	update_curr_dl(rq);
>  	__dequeue_task_dl(rq, p, flags);
> +
> +	if (p->on_rq == TASK_ON_RQ_MIGRATING)
> +		sub_running_bw(&p->dl, &rq->dl);
> +
> +	if (flags & DEQUEUE_SLEEP)
> +		sub_running_bw(&p->dl, &rq->dl);
>  }
>  
>  /*
> @@ -1501,7 +1530,9 @@ static int push_dl_task(struct rq *rq)
>  	}
>  
>  	deactivate_task(rq, next_task, 0);
> +	sub_running_bw(&next_task->dl, &rq->dl);
>  	set_task_cpu(next_task, later_rq->cpu);
> +	add_running_bw(&next_task->dl, &later_rq->dl);
>  	activate_task(later_rq, next_task, 0);
>  	ret = 1;
>  
> @@ -1589,7 +1620,9 @@ static void pull_dl_task(struct rq *this_rq)
>  			resched = true;
>  
>  			deactivate_task(src_rq, p, 0);
> +			sub_running_bw(&p->dl, &src_rq->dl);
>  			set_task_cpu(p, this_cpu);
> +			add_running_bw(&p->dl, &this_rq->dl);
>  			activate_task(this_rq, p, 0);
>  			dmin = p->dl.deadline;
>  
> @@ -1695,6 +1728,9 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
>  	if (!start_dl_timer(p))
>  		__dl_clear_params(p);
>  
> +	if (task_on_rq_queued(p))
> +		sub_running_bw(&p->dl, &rq->dl);
> +
>  	/*
>  	 * Since this might be the only -deadline task on the rq,
>  	 * this is the right place to try to pull some other one
> @@ -1712,6 +1748,7 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
>   */
>  static void switched_to_dl(struct rq *rq, struct task_struct *p)
>  {
> +	add_running_bw(&p->dl, &rq->dl);
>  
>  	/* If p is not queued we will update its parameters at next wakeup. */
>  	if (!task_on_rq_queued(p))

Don't we also need to remove bw in task_dead_dl()?

Thanks,

- Juri

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