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

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

[Mitchell Erblich]

Ingo Molnar and group,

If their is a single RT task on this rq, then why not just reset the
timeslice and return??? Just MAYBE decreasing re-scheduling
overhead..

Thus,,,,

After
    p->time_slice = static_prio_timeslice(p->static_prio);

Why isn't their a check like
    if  (rq->nr_running == 1)
         return;

Which world remove the need for any recheduling
or requeue'ing...

Mitchell Erblich
FYI: below is believed to be a snap of the current/ orig func
-----------------------------


+static void task_tick_rt(struct rq *rq, struct task_struct *p)
+{
+ /*
+  * RR tasks need a special form of timeslice management.
+  * FIFO tasks have no timeslices.
+  */
+ if (p->policy != SCHED_RR)
+  return;
+
+ if (--p->time_slice)
+  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);
+}

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

[Ingo Molnar]

* Mitchell Erblich <erblichs@earthlink.net> wrote:

> After
>     p->time_slice = static_prio_timeslice(p->static_prio);
> 
> Why isn't their a check like
>     if  (rq->nr_running == 1)
>          return;
> 
> Which world remove the need for any recheduling or requeue'ing...

your change is a possible optimization, but this is a pretty rare 
codepath because the overwhelming majority of RT apps uses SCHED_FIFO. 
Plus, the time_slice going down to 0 is a relatively rare event even for 
SCHED_RR tasks. And if we have only a single SCHED_RR task, why is it 
SCHED_RR to begin with? So this is on several levels an uncommon 
workload and by adding a check like that we'd increase the codesize. But 
... no strong feelings against this optimization - if you send a proper 
patch we can apply it, it certainly makes sense from a logic POV.

	Ingo

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

[Dmitry Adamushko]

On 08/08/07, Ingo Molnar <mingo@elte.hu> wrote:
>
> * Mitchell Erblich <erblichs@earthlink.net> wrote:
>
> > After
> >     p->time_slice = static_prio_timeslice(p->static_prio);
> >
> > Why isn't their a check like
> >     if  (rq->nr_running == 1)
> >          return;
> >
> > Which world remove the need for any recheduling or requeue'ing...
>
> your change is a possible optimization, but this is a pretty rare
> codepath because the overwhelming majority of RT apps uses SCHED_FIFO.
> Plus, the time_slice going down to 0 is a relatively rare event even for
> SCHED_RR tasks. [ ... ]

I doubt it's a worth optimization. It's a rare codepath and, moreover,
this optimization is sub-optimal. e.g. the rest of tasks that
contributed to 'rq->nr_running > 1' could be SCHED_NORMAL (or of lower
rt_prio) and so resched_task() is useless as well.

I guess, the following thing would do a better job:

- we do need reschedule() _only_  if there are other task on the
_same_ queue (for this prio level) :

(1) if there is a task with a higher prio, resched_task() would be
called in other place;
(2) if there are tasks of lower prio, we don't care.

I may be wrong, but at the first glance it looks like the following
trick would do the job.. no?

p.s. even if it's ok, it's still a rare codepath (although, it adds
some code to the rare path.. so the drawback is only code-size, I
guess).

--- kernel/sched_rt-prev.c      2007-08-09 09:55:10.000000000 +0200
+++ kernel/sched_rt.c   2007-08-09 09:58:53.000000000 +0200
@@ -207,6 +207,11 @@ static void task_tick_rt(struct rq *rq,
                return;

        p->time_slice = static_prio_timeslice(p->static_prio);
+
+       /* We are the only element on the queue. */
+       if (p->run_list.prev == p->run_list.next)
+               return;
+
        set_tsk_need_resched(p);

        /* put it at the end of the queue: */


-- 
Best regards,
Dmitry Adamushko

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

[Ingo Molnar]

* Dmitry Adamushko <dmitry.adamushko@gmail.com> wrote:

> I guess, the following thing would do a better job:
> 
> - we do need reschedule() _only_ if there are other task on the _same_ 
> queue (for this prio level) :

ah, indeed.

> --- kernel/sched_rt-prev.c      2007-08-09 09:55:10.000000000 +0200
> +++ kernel/sched_rt.c   2007-08-09 09:58:53.000000000 +0200
> @@ -207,6 +207,11 @@ static void task_tick_rt(struct rq *rq,
>                 return;
> 
>         p->time_slice = static_prio_timeslice(p->static_prio);
> +
> +       /* We are the only element on the queue. */
> +       if (p->run_list.prev == p->run_list.next)
> +               return;
> +

i've applied your patch - could you please send a SOB line?

	Ingo

Re: Question: RT schedular : task_tick_rt(struct rq *rq, struct task_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@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 = {

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

[Dmitry Adamushko]

On 09/08/07, Ingo Molnar <mingo@elte.hu> wrote:
>
> FYI, that's the patch i applied:

Thanks. Added my SOB below.

>
> --------------------------->
> 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>


Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>


> ---
>  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 = {
>


-- 
Best regards,
Dmitry Adamushko