Re: [PATCH] small optimization to update_curr_rt

From: Peter Zijlstra
Date: Mon Nov 03 2008 - 05:21:52 EST


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@xxxxxxxxx>

> Signed-off-by: Dimitri Sivanich <sivanich@xxxxxxx>
>
> --
>
> 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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