linux-kernel - Re: [PATCH] sched: update cpupri for runqueue when its priority changes

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Date:	Sun, 19 Jun 2011 16:33:32 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Yong Zhang <yong.zhang0@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: update cpupri for runqueue when its priority changes

On Sun, Jun 19, 2011 at 1:11 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>> There are two cases that NULL is returned in _pick_next_task_rt(), it is the
>> second case, after checking rt_rq->rt_nr_running, that is captured, and if
>> NULL is returned in the second case, the CPU priority does change.
>
> The two cases are:
>
> 1) no rt task exists
> 2) the runqueue is throttled.
>
> We already talked about the throttled case. The case where no rt task
> exists means that the last rt task has been dequeued. When that happens,
> the cpupri is updated then. I don't see any bug. There's no need to
> update cpupri at this point.
>

In case 2) , the CPU will be held then by non-RT task, and we have to
update priority.

I understand 1) and 2) to be, there has RT tasks on runqueue but they
could not run at the moment.

If throttled RQ == no RT tasks, please drop this patch directly.

>>
>> In another scenario that has little with {en, de}queue, as shown by
>> requeue_task_rt(), the CPU priority will change if other RT tasks exist.
>>
>
> The requeue_task_rt() does not change the priority of the CPU. It just
> updates the task in its order of placement in the queue of other tasks
> of the same priority.
>

You are right, updating CPU priority for requeue is unnecessary.

thanks
           Hillf
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