Re: Question: RT schedular : task_tick_rt(struct rq *rq, structtask_struct *p) : decreases overhead when rq->nr_running == 1

[Ingo Molnar]

FYI, that's the patch i applied:

--------------------------->
Subject: sched: optimize task_tick_rt() a bit
From: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx>

Mitchell Erblich suggested a change to not requeue SCHED_RR
tasks if there's only a single task on the runqueue, by
checking for rq->nr_running == 1.

provide a more efficient implementation of that, to check that
particular RT priority-queue only.

[ From: mingo@xxxxxxx ]

Also first requeue the task then set need_resched - results
in slightly better machine-instruction ordering. Also clean
up the code a bit.

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 kernel/sched_rt.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

Index: linux/kernel/sched_rt.c
===================================================================
--- linux.orig/kernel/sched_rt.c
+++ linux/kernel/sched_rt.c
@@ -207,10 +207,15 @@ static void task_tick_rt(struct rq *rq, 
 		return;
 
 	p->time_slice = static_prio_timeslice(p->static_prio);
-	set_tsk_need_resched(p);
 
-	/* put it at the end of the queue: */
-	requeue_task_rt(rq, p);
+	/*
+	 * Requeue to the end of queue if we are not the only element
+	 * on the queue:
+	 */
+	if (p->run_list.prev != p->run_list.next) {
+		requeue_task_rt(rq, p);
+		set_tsk_need_resched(p);
+	}
 }
 
 static struct sched_class rt_sched_class __read_mostly = {
-
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