linux-kernel - Re: [PATCH v2 2/2] sched/rt: Remove unnecessary condition in push_rt

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Date:   Wed, 15 Feb 2017 21:36:53 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     <peterz@...radead.org>, <mingo@...nel.org>,
        <linux-kernel@...r.kernel.org>, <juri.lelli@...il.com>,
        <kernel-team@....com>
Subject: Re: [PATCH v2 2/2] sched/rt: Remove unnecessary condition in
 push_rt_task()

On Thu, 16 Feb 2017 11:05:54 +0900
Byungchul Park <byungchul.park@....com> wrote:

> Once pick_next_pushable_task(rq) returns a task, it guarantees that
> the task's cpu is rq->cpu, so task_cpu(next_task) is always rq->cpu if
> task == next_task. Remove the redundant condition and make code simpler.
> 
> pick_next_pushable_task(rq) has BUG_ON(rq_cpu != task_cpu(task)) when
> it returns a task other than NULL, which means that task_cpu(task) must
> be rq->cpu. So if task == next_task, then task_cpu(next_task) must be
> rq->cpu as well.
> 
> By this patch, unnecessary one branch and two LOAD operations in 'if'
> statement can be avoided.
> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/sched/rt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4101f9d..5abd9a52 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1820,7 +1820,7 @@ static int push_rt_task(struct rq *rq)
>  		 * pushing.
>  		 */
>  		task = pick_next_pushable_task(rq);
> -		if (task_cpu(next_task) == rq->cpu && task == next_task) {
> +		if (task == next_task) {

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve

>  			/*
>  			 * The task hasn't migrated, and is still the next
>  			 * eligible task, but we failed to find a run-queue