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

    FYI, that's the patch i applied:

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

    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@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: Ingo Molnar <mingo@elte.hu>
    ---
    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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