sched: optimize task_tick_rt() a bit

Mitchell Erblich suggested a quality-of-implementation 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@elte.hu ]

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: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 8 insertions, 3 deletions

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index dcdcad632fd9..4b87476a02d0 100644
--- a/kernel/sched_rt.c
+++ b/kernel/sched_rt.c
@@ -207,10 +207,15 @@ static void task_tick_rt(struct rq *rq, struct task_struct *p)
                 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 = {