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

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Message-ID: <20170223101123.GE3817@X58A-UD3R>
Date:   Thu, 23 Feb 2017 19:11:23 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     <peterz@...radead.org>, <mingo@...nel.org>
CC:     <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 Thu, Feb 23, 2017 at 03:14:59PM +0900, 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.

To be honest, I think this patch is more valuable when used with
removing unnecessary hrtimer interrupt for deadline control, but
not that much without it.

I just wonder what you think.

> 
> 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);
> +	if (!task_on_rq_queued(p))
>  		goto unlock;
> -	}
>  
>  #ifdef CONFIG_SMP
>  	if (unlikely(!rq->online)) {
> -- 
> 1.9.1