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

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Date:	Thu, 12 Jan 2012 12:09:27 +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 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, 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.

Thanks,
Rakib
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