linux-kernel - Re: [PATCH] small optimization to update_curr

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Date:	Mon, 03 Nov 2008 11:22:04 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dimitri Sivanich <sivanich@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] small optimization to update_curr_rt

On Fri, 2008-10-31 at 08:03 -0500, Dimitri Sivanich wrote:
> A very minor improvement, but might it be better to check sched_rt_runtime(rt_rq)
> before taking the rt_runtime_lock?

Yes, I think its ok to do so.

Like pointed out in the other thread, there are two races:

 - sched_rt_runtime() going to RUNTIME_INF, and that will be handled
   properly by sched_rt_runtime_exceeded()

 - sched_rt_runtime() going to !RUNTIME_INF, and here we can miss an
   accounting cycle, but I don't think that is something to worry too
   much about.

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

> Signed-off-by: Dimitri Sivanich <sivanich@....com>
> 
> --
> 
>  kernel/sched_rt.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> Index: linux/kernel/sched_rt.c
> ===================================================================
> --- linux.orig/kernel/sched_rt.c	2008-10-22 16:10:03.000000000 -0500
> +++ linux/kernel/sched_rt.c	2008-10-31 07:57:19.000000000 -0500
> @@ -537,13 +537,13 @@ static void update_curr_rt(struct rq *rq
>  	for_each_sched_rt_entity(rt_se) {
>  		rt_rq = rt_rq_of_se(rt_se);
>  
> -		spin_lock(&rt_rq->rt_runtime_lock);
>  		if (sched_rt_runtime(rt_rq) != RUNTIME_INF) {
> +			spin_lock(&rt_rq->rt_runtime_lock);
>  			rt_rq->rt_time += delta_exec;
>  			if (sched_rt_runtime_exceeded(rt_rq))
>  				resched_task(curr);
> +			spin_unlock(&rt_rq->rt_runtime_lock);
>  		}
> -		spin_unlock(&rt_rq->rt_runtime_lock);
>  	}
>  }
>  
--
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