RE: [PATCH 1/1] scheduler: minor improvement topick_next_highest_task_rt in linux-3.3

[Michael J. Wang]

> From: Peter Zijlstra [mailto:peterz@xxxxxxxxxxxxx]
> Sent: Wednesday, March 21, 2012 2:07 AM
> To: Michael J. Wang
> Cc: Yong Zhang; mingo@xxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> rostedt@xxxxxxxxxxx
> Subject: RE: [PATCH 1/1] scheduler: minor improvement to
> pick_next_highest_task_rt in linux-3.3
> 
> On Wed, 2012-03-21 at 07:49 +0000, Michael J. Wang wrote:
> > > > Should I reformat my patch and send it again?
> > >
> > > It'll be better, and I think Peter/Ingo will happy with it.
> > >
> >
> > OK.  I will resend now.  Thanks for all your help!
> 
> The resend didn't include the more details thing, so I fudged it by
> hand. The queued thing now looks like this:
> 

OK.  Thanks.  I was afraid the details were too verbose when the fix
was obvious to the experts.  Anyways, I now know the format you
are expecting, so I will do better next time.

Michael

> ---
> Subject: sched, rt: Minor improvement to pick_next_highest_task_rt
> From: Michael J Wang <mjwang@xxxxxxxxxxxx>
> Date: Mon, 19 Mar 2012 22:26:19 +0000
> 
> Avoid extra work by continuing on to the next rt_rq if the highest
> prio task in current rt_rq is the same priority as our candidate
> task.
> 
> More detailed explanation:  if next is not NULL, then we have found a
> candidate task, and its priority is next->prio.  Now we are looking
> for an even higher priority task in the other rt_rq's.  idx is the
> highest priority in the current candidate rt_rq.  In the current 3.3
> code, if idx is equal to next->prio, we would start scanning the tasks
> in that rt_rq and replace the current candidate task with a task from
> that rt_rq.  But the new task would only have a priority that is equal
> to our previous candidate task, so we have not advanced our goal of
> finding a higher prio task.  So we should avoid the extra work by
> continuing on to the next rt_rq if idx is equal to next->prio.
> 
> Signed-off-by: Michael J Wang <mjwang@xxxxxxxxxxxx>
> Acked-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Reviewed-by: Yong Zhang <yong.zhang0@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Link:
> http://lkml.kernel.org/r/2EF88150C0EF2C43A218742ED384C1BC0FC83D6B@IRVEX
> CHMB08.corp.ad.broadcom.com
> ---
>  kernel/sched/rt.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1428,7 +1428,7 @@ static struct task_struct *pick_next_hig
>  next_idx:
>  		if (idx >= MAX_RT_PRIO)
>  			continue;
> -		if (next && next->prio < idx)
> +		if (next && next->prio <= idx)
>  			continue;
>  		list_for_each_entry(rt_se, array->queue + idx, run_list) {
>  			struct task_struct *p;
> 


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