linux-kernel - Re: [Question] sched: Should nr_uninterruptible be decremented in ttwu_do

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Date:	Mon, 16 Jan 2012 17:22:07 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Rakib Mullick <rakib.mullick@...il.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [Question] sched: Should nr_uninterruptible be decremented in
 ttwu_do_activate()?

On 01/16/2012 04:27 PM, Rakib Mullick wrote:

> On Mon, Jan 16, 2012 at 1:53 PM, Michael Wang
> <wangyun@...ux.vnet.ibm.com> wrote:
>> On 01/13/2012 01:08 AM, Rakib Mullick wrote:
>>
>>> On Thu, Jan 12, 2012 at 1:25 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>>>> On Thu, 2012-01-12 at 12:09 +0600, Rakib Mullick wrote:
>>>>> On Wed, Jan 11, 2012 at 11:29 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>>>>>> On Wed, 2012-01-11 at 23:22 +0600, Rakib Mullick wrote:
>>>>>>> Hello all,
>>>>>>>
>>>>>>> In ttwu_do_activate(), we're decrementing nr_uninterruptible if
>>>>>>> p->sched_contributes_to_load (for SMP=y). But, we're also decrementing
>>>>>>> nr_uninterruptible from activate_task at the same path. Why we're
>>>>>>> doing it twice for a single task activation path?
>>>>>>
>>>>>> activate_task() does:
>>>>>>
>>>>>>  if (task_contributes_to_load(p))
>>>>>>   rq->nr_uninterruptible--;
>>>>>>
>>>>>> Now task_contributes_to_load() reads like:
>>>>>>
>>>>>> #define task_contributes_to_load(task)  \
>>>>>>                                ((task->state & TASK_UNINTERRUPTIBLE) != 0 && \
>>>>>>                                 (task->flags & PF_FREEZING) == 0)
>>>>>>
>>>>>> which will be false, since we've set TASK_WAKING.
>>>>>
>>>>> Enough confusing. TASK_WAKING will be set when called from
>>>>> try_to_wake_up(). ttwu_do_activate() gets called from other places:
>>>>> scheduler_ipi() and sched_ttwu_pending() (at the time of cpu goes
>>>>> down). TASK_WAKING will be not set at that time,
>>>>
>>>> Yes it will be, the only way to get on that list is throught
>>>> ttwu_queue_remote() at which point tasks are TASK_WAKING.
>>>>
>>>>>  moreover it is
>>>>> possible that, task has p->sched_contributes_to_load is set and latter
>>>>> on gets wake up by sched_ttwu_pending/scheduler_ipi() call.
>>>>
>>>> That's the entire point. But all ways to ttwu_queue_remote() explicitly
>>>> set ->sched_contributes_to_load.
>>>
>>> That might be the case for scheduler_ipi(), but when
>>> sched_ttwu_pending() gets called when a cpu goes down, all tasks from
>>> wake_list of that cpu has been moved without TASK_WAKING is set. For a
>>
>>
>> I think the task in rq->wake_list should already have state:TASK_WAKING,
>> because it's a wake list.
>>
> But, what I got by means of TASK_WAKING is this task is about to RUN,
> very soon it'll have TASK_RUNNING state. And, if I hadn't miss any
> portion of code, then rq->wake_list doesn't have TASK_WAKING state.
> 

I saw this is the way to enqueue wake_list:

try_to_wake_up --> p->state = TASK_WAKING; --> ttwu_queue -->
ttwu_queue_remote --> llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list)

BTW, I'm just start to learn scheduler, may be I'm wrong, let's find out
the right answer :)

Thanks,
Michael Wang

> Thanks,
> Rakib
> 


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