linux-kernel - Re: [PATCH] sched: Check nr_running before calling pick_next

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Date:	Sat, 2 Jul 2011 20:26:56 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH] sched: Check nr_running before calling pick_next_task in schedule().

On Sat, Jul 2, 2011 at 3:51 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sat, 2011-07-02 at 00:41 +0600, Rakib Mullick wrote:
>> Currently at schedule(), when we call pick_next_task we don't check
>> whether current rq is empty or not. Since idle_balance can fail,
>> its nice to check whether we really have any task on rq or not. If
>> not, we can call idle_sched_class.pick_next_task straight.
>>
>> Signed-off-by: Rakib Mullick <rakib.mullick@...il.com>
>> ---
>>
>> diff --git a/kernel/sched.c b/kernel/sched.c
>> index 5925275..a4f4f58 100644
>> --- a/kernel/sched.c
>> +++ b/kernel/sched.c
>> @@ -4273,7 +4273,14 @@ need_resched:
>>               idle_balance(cpu, rq);
>>
>>       put_prev_task(rq, prev);
>> -     next = pick_next_task(rq);
>> +     /*  Since idle_balance can fail, its better to check rq->nr_running.
>> +      *  Otherwise we can call idle_sched_class.pick_next_task straight,
>> +      *  cause we need to do some accounting.
>> +      */
>> +     if (likely(rq->nr_running))
>> +             next = pick_next_task(rq);
>> +     else
>> +             next = idle_sched_class.pick_next_task(rq);
>>       clear_tsk_need_resched(prev);
>>       rq->skip_clock_update = 0;
>
> Why!?
>
> You're making the fast path -- picking a task -- slower by adding a
> branch, and making the slow path -- going into idle -- faster. That
> seems backwards at best.
>
Well, yes - branching seems definitely have some side effects.
Thinking from UP's perspective, it will only hit slow path -- going
into idle. In that case, that likely branch will just fail. And on an
UP system that slow path -- going into idle -- is the only way, taking
the fast path (trying picking a task) isn't the right thing, isn't it?

> You've completely failed to provide any sort of rationale for the patch
> nor did you provide a use-case with performance numbers. This just isn't
> making much sense at all.
>
Regarding taking branch, it needs performance numbers - I agree. But,
I'm not quite agree that - it's not just making any sense. Will try to
do some performance testing on it.


Thanks,
Rakib
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