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    SubjectRe: [Question] sched: Should nr_uninterruptible be decremented in ttwu_do_activate()?
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    On Thu, Jan 12, 2012 at 1:25 PM, Peter Zijlstra <a.p.zijlstra@chello.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@chello.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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