Re: minor improvement to pick_next_highest_task_rt ?

From: Yong Zhang
Date: Mon Mar 19 2012 - 02:32:47 EST


Cc'ing more people.

And comments below.

On Fri, Mar 16, 2012 at 01:22:56AM +0000, Michael J. Wang wrote:
> Hi RT Scheduler experts,
>
> I was studying pick_next_highest_task_rt() and was wondering if this is a valid improvement:
>
> --- rt.c-3.3-rc7 2012-03-15 17:53:27.774190199 -0700
> +++ rt.c 2012-03-15 17:53:44.541979403 -0700
> @@ -1403,7 +1403,7 @@
> 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;
>
>
> My reasoning is: 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-rc7 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 shouldn't we just skip that rt_rq if next->prio is less than *or equal to* idx ?

Yeah, I think this make sense.

But you should remake your patch according to
Documentation/SubmittingPatches.

Thanks,
Yong

>
> I know this is just a minor improvement and probably results in no measurable performance gain. But it just seems more correct this way. (Or if it is not correct, maybe I'll learn something :-)
>
> I do not subscribe to the LKML (but I have read the FAQ), so I would appreciate it if you can cc me on your responses.
>
> Thanks,
> Michael
>
>
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