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

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Message-ID: <CADZ9YHiGjjGJjxKj4WKWydqAz0zxFwY5=LsYBKhEREuZm=bKpg@mail.gmail.com>
Date:	Thu, 12 Jan 2012 23:08:53 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	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 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
particular task it might be possible that when it ran previously it
had p->sched_contributes_to_load is set. Latter, this task's cpu has
been put down and calls sched_ttwu_pending(), then for that task
p->sched_contributes_to_load is set and TASK_WAKING is not set.
Couldn't be happen?

Thanks,
Rakib
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