[RFC][PATCH 04/10] sched: Change pick_next_task_rt from unlikely to likely

[Steven Rostedt]

From: Steven Rostedt <srostedt@xxxxxxxxxx>

The if (unlikely(!rt_rq->rt_nr_running)) test in pick_next_task_rt()
tests if there is another rt task ready to run. If so, then pick it.

In most systems, only one RT task runs at a time most of the time.
Running the branch unlikely annotator profiler on a system doing average
work "running firefox, evolution, xchat, distcc builds, etc", it showed the
following:

 correct incorrect  %        Function                  File              Line
 ------- ---------  -        --------                  ----              ----
  324344 135104992  99 _pick_next_task_rt             sched_rt.c           1064

99% of the time the condition is true. When an RT task schedules out,
it is unlikely that another RT task is waiting to run on that same run queue.

Cc:Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Gregory Haskins <ghaskins@xxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 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 7a5c4db..a249ae3 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -1062,7 +1062,7 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
 
 	rt_rq = &rq->rt;
 
-	if (unlikely(!rt_rq->rt_nr_running))
+	if (likely(!rt_rq->rt_nr_running))
 		return NULL;
 
 	if (rt_rq_throttled(rt_rq))
-- 
1.7.2.3


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/