linux-kernel - Re: [sched/rt] Optimization of function pull_rt

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Message-ID: <1334592379.28106.4.camel@gandalf.stny.rr.com>
Date:	Mon, 16 Apr 2012 12:06:19 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [sched/rt] Optimization of function pull_rt_task()

On Sun, 2012-04-15 at 23:45 +0400, Kirill Tkhai wrote:
> The condition (src_rq->rt.rt_nr_running) is weak because it doesn't
> consider the cases when src_rq has only processes bound to it (when
> single cpu is allowed). It may be running kernel thread like
> migration/x etc.
> 
> So it's better to use more stronger condition which is able to exclude
> above conditions. The function has_pushable_tasks() complitely does
> this. A task may be pullable for another cpu rq only if he is pushable
> for his own queue.

I considered this before, and for some reason I never did the change.
I'll have to think about it. It seems like this would be the obvious
case, but I think there was something not so obvious that caused issues.
But I don't remember what it was.

I'll have to rethink this again.

Thanks,

-- Steve

> 
> Signed-off-by: Kirill Tkhai <tkhai@...dex.ru>
> ---
>  kernel/sched/rt.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index c5565c3..61e3086 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1729,7 +1729,7 @@ static int pull_rt_task(struct rq *this_rq)
>  		/*
>  		 * Are there still pullable RT tasks?
>  		 */
> -		if (src_rq->rt.rt_nr_running <= 1)
> +		if (!has_pushable_tasks(src_rq))
>  			goto skip;
>  
>  		p = pick_next_highest_task_rt(src_rq, this_cpu);
> 


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