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

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Date:	Fri, 17 Jan 2014 16:06:45 +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 03:26 PM, Peter Zijlstra wrote:
> On Fri, Jan 17, 2014 at 10:04:04AM +0100, Daniel Lezcano wrote:
>> @@ -2679,11 +2715,8 @@ need_resched:
>>
>>   	pre_schedule(rq, prev);
>>
>> -	if (unlikely(!rq->nr_running))
>> -		rq->idle_stamp = idle_balance(rq) ? 0 : rq_clock(rq);
>> -
>>   	put_prev_task(rq, prev);
>> -	next = pick_next_task(rq);
>> +	next = pick_next_task_or_idle(rq);
>>   	clear_tsk_need_resched(prev);
>>   	clear_preempt_need_resched();
>>   	rq->skip_clock_update = 0;
>
> I have vague memories that we need to have idle_balance() before
> put_prev_task(), but I can't recollect why this would be so.
>
> That said, if I resurrect these patches:
>
>    https://lkml.org/lkml/2012/6/14/271
>
> 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  ?


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