linux-kernel - Re: [PATCH 4/4] sched: Idle task shortcut optimization

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Date:	Fri, 17 Jan 2014 16:26:13 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linaro-kernel@...ts.linaro.org, alex.shi@...aro.org
Subject: Re: [PATCH 4/4] sched: Idle task shortcut optimization

On 01/17/2014 04:23 PM, Peter Zijlstra wrote:
> On Fri, Jan 17, 2014 at 04:06:45PM +0100, Daniel Lezcano wrote:
>>> I suppose we could write something like:
>>>
>>> struct task_struct *pick_next_task(struct rq *rq, struct task_struct *prev)
>>> {
>>> 	const struct sched_class *class;
>>> 	struct task_struct *p;
>>>
>>> again:
>>> 	if (likely(rq->nr_running)) {
>>>
>>> 		if (likely(rq->nr_running == rq->cfs.h_nr_running))
>>> 			return fair_sched_class.pick_next_task(rq, prev);
>>>
>>> 		for_each_class(class) {
>>> 			p = class->pick_next_task(rq, prev);
>>> 			if (p)
>>> 				return p;
>>> 		}
>>> 	}
>>>
>>> 	if (idle_balance(rq))
>>> 		goto again;
>>>
>>> 	rq->idle_stamp = rq_clock(rq);
>>>
>>> 	return idle_sched_class.pick_next_task(rq, prev);
>>> }
>>>
>>> Which keeps idle_balance() before put_prev_task(), and by using
>>> idle_sched_clas.pick_next_task() doesn't rape the idle class interface
>>> like you did :-)
>>
>> But put_prev_task is called before pick_next_task, so idle_balance() is
>> called after now, no  ?
>
> No, put_prev_task() is called by the pick_next_task() that returns a
> task. So in the idle case above, the idle_sched_class.pick_next_task()
> will do the required put_prev_task().

Ah, ok. Let me try it.



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