linux-kernel - Re: [PATCH RESEND 2/2] sched/rt: Add check_preempt_equal_prio() logic in pick_next_task

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Date:	Tue, 3 Feb 2015 22:17:38 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Xunlei Pang <xlpang@....com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...il.com>,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: Re: [PATCH RESEND 2/2] sched/rt: Add check_preempt_equal_prio()
 logic in pick_next_task_rt()

On Wed,  4 Feb 2015 09:12:21 +0800
Xunlei Pang <xlpang@....com> wrote:

> From: Xunlei Pang <pang.xunlei@...aro.org>
> 
> check_preempt_curr() doesn't call sched_class::check_preempt_curr
> when the class of current is a higher level. So if there is a DL
> task running when doing this for RT, check_preempt_equal_prio()
> will definitely miss, which may result in some response latency
> for this RT task if it is pinned and there're some same-priority
> migratable rt tasks already queued.
> 
> We should do the similar thing in select_task_rq_rt() when first
> picking rt tasks after running out of DL tasks.
> 
> This patch tackles the issue by peeking the next rt task(RT1), and
> if find RT1 migratable, just requeue it to the tail of the rq using
> requeue_task_rt(rq, p, 0). In this way:
> - If there do have another rt task(RT2) with the same priority as
>   RT1, RT2 will finally be picked as the running task. While RT1
>   will be pushed onto another cpu via RT1's post_schedule(), as
>   RT1 is migratable. The difference from check_preempt_equal_prio()
>   here is that we just don't care whether RT2 is migratable.
> 
> - Otherwise, if there's no rt task with the same priority as RT1,
>   RT1 will still be picked as the running task after the requeuing.
> 
> Signed-off-by: Xunlei Pang <pang.xunlei@...aro.org>
> ---
>  kernel/sched/rt.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4dacb6e..b2385ee 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1477,6 +1477,21 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev)
>  
>  	put_prev_task(rq, prev);
>  
> +#ifdef CONFIG_SMP
> +		/*
> +		 * If there's a running deadline task, check_preempt_curr()
> +		 * doesn't invoke check_preempt_curr_rt() for rt tasks, so
> +		 * we can do it here.
> +		 */

Why the strange indentation?

> +		if (prev->sched_class == &dl_sched_class &&
> +		    rq->rt.rt_nr_total > 1) {
> +			p = _pick_next_task_rt(rq, 1); /* peek only */

I hate the "peek only". Just split the function into two, where you
have something like check_next_task(rq) which does your "peek only"
and the __pick_next_task_rt() calls check_next_task() first and then
runs the rest of the code.

-- Steve

> +			if (p->nr_cpus_allowed != 1 &&
> +			    cpupri_find(&rq->rd->cpupri, p, NULL))
> +				requeue_task_rt(rq, p, 0);
> +		}
> +#endif
> +
>  	p = _pick_next_task_rt(rq, 0);
>  
>  	/* The running task is never eligible for pushing */

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