linux-kernel - Re: [PATCH v2 2/6] sched/rt: Optimize select_task_rq

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Date:	Tue, 04 Nov 2014 20:21:37 +0800
From:	"Hillf Danton" <hillf.zj@...baba-inc.com>
To:	"'pang.xunlei'" <pang.xunlei@...aro.org>
Cc:	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"'Ingo Molnar'" <mingo@...hat.com>,
	"'Peter Zijlstra'" <peterz@...radead.org>,
	"'Steven Rostedt'" <rostedt@...dmis.org>,
	"'Juri Lelli'" <juri.lelli@...il.com>
Subject: Re: [PATCH v2 2/6] sched/rt: Optimize select_task_rq_rt() for non-RT curr task

> 
> When selecting the cpu for a waking RT task, if curr is a non-RT
> task which is bound only on this cpu, then we can give it a chance
> to select a different cpu(definitely an idle cpu if existing) for
> the RT task to avoid curr starving.
> 
> Signed-off-by: pang.xunlei <pang.xunlei@...aro.org>
> ---
>  kernel/sched/rt.c |   10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index da6922e..dc1f7f0 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1340,6 +1340,11 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
>  	 * runqueue. Otherwise simply start this RT task
>  	 * on its current runqueue.
>  	 *
> +	 * If the current task on @p's runqueue is a non-RT task,
> +	 * and this task is bound on current runqueue, then try to
> +	 * see if we can wake this RT task up on a different runqueue,
> +	 * we will definitely find an idle cpu if there is any.
> +	 *
>  	 * We want to avoid overloading runqueues. If the woken
>  	 * task is a higher priority, then it will stay on this CPU
>  	 * and the lower prio task should be moved to another CPU.
> @@ -1356,9 +1361,8 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
>  	 * This test is optimistic, if we get it wrong the load-balancer
>  	 * will have to sort it out.
>  	 */
> -	if (curr && unlikely(rt_task(curr)) &&
> -	    (curr->nr_cpus_allowed < 2 ||
> -	     curr->prio <= p->prio)) {
> +	if (curr && unlikely(curr->nr_cpus_allowed < 2 ||
> +				curr->prio <= p->prio)) {

Nack, it is no meaning to compare apple against orange.

Hillf
>  		int target = find_lowest_rq(p);
> 
>  		if (target != -1)
> --
> 1.7.9.5

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