linux-kernel - Re: [PATCH v2] sched/deadline: fix start high-res preemption tick for a non-leftmost task

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Message-ID: <546B8251.5080805@arm.com>
Date:	Tue, 18 Nov 2014 17:30:57 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Kirill Tkhai <ktkhai@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched/deadline: fix start high-res preemption tick
 for a non-leftmost task

Hi,

On 18/11/14 02:32, Wanpeng Li wrote:
> Queued ticks are scheduled to match the budget, which means the budget
> is overall consumed and the dl task should be throttled. 
> 
> Dl task will be replenished immediately if fail to start a dl timer.
> So the curr maybe not the left most dl task in the rb tree any more 
> after this immediately replenished since reschedule is needed.
> 
> The parameter of task_tick_dl() queued == 1 means that hrtick is fired.
> hrtick() => task_tick_dl( , ,1), so p->dl.runtime should be <= 0 if
> queued == 1. If replenished immediately(fail to start dl timer),
> hrtick is start for task w/ queued == 1 && p->dl.runtime > 0 && p is 
> not the left most task.
> 
> Start high-res preemption tick for this upcoming rescheduled dl task
> is not correct. This patch fix it by not starting high-res preemption 
> tick for a non-leftmost dl task.
> 
> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
> ---
> v1 -> v2:
>  * add a more explanatory changelog 
>  * add a comment just above the check
> 
>  kernel/sched/deadline.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index e5db8c6..981479a 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1090,7 +1090,18 @@ static void task_tick_dl(struct rq *rq, struct task_struct *p, int queued)
>  {
>  	update_curr_dl(rq);
>  
> -	if (hrtick_enabled(rq) && queued && p->dl.runtime > 0)
> +	/*
> +	 * Queued tick means that hrtick is fired, when the budget is
> +	 * consumed the task has to be throttled. However, task will
> +	 * be replenished immediately if fail to start a dl timer, in
> +	 * addition, the task maybe not the left most any more. Don't
> +	 * start high-res preemption tick for the task who is not the
> +	 * left most. The high-res preemption tick will be started for
> +	 * this task just when it can be picked in pick_next_task_dl()
> +	 * again.
> +	 */
> +	if (hrtick_enabled(rq) && queued && p->dl.runtime > 0 &&
> +	    is_leftmost(p, &rq->dl))
>  		start_hrtick_dl(rq, p);
>  }
>  
> 

Acked-by: Juri Lelli <juri.lelli@....com>

Thanks,

- Juri

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