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    SubjectRe: [PATCH RFC] reduce runqueue lock contention
    On 06/21/10 06:04, Peter Zijlstra wrote:
    > On Mon, 2010-06-21 at 12:54 +0200, Peter Zijlstra wrote:
    >>> It looses the ttwu task_running() check, as I must admit I'm not quite
    >>> sure what it does.. Ingo?
    > I think I figured out what its for, its for when p is prev in schedule()
    > after deactivate_task(), we have to call activate_task() it again, but
    > we cannot migrate the task because the CPU its on is still referencing
    > it.

    I have not been able to make sense of the task_running() check in
    try_to_wake_up(), even with that clue. The try_to_wake_up() code in
    question is:

    rq = task_rq_lock(p, &flags);
    if (!(p->state & state))
    goto out;

    if (p->se.on_rq)
    goto out_running;

    cpu = task_cpu(p);
    orig_cpu = cpu;

    #ifdef CONFIG_SMP
    if (unlikely(task_running(rq, p)))
    goto out_activate;

    The relevent code in schedule() executes with the rq lock held (many
    lines left out to emphasize the key lines):


    if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {

    deactivate_task(rq, prev, DEQUEUE_SLEEP);

    if (likely(prev != next)) {
    rq->curr = next;
    context_switch(rq, prev, next); /* unlocks the rq */
    } else

    If (p->se.on_rq) can becomes false due to deactivate_task()
    then task_running() will also become false while the rq lock is still
    held (either via "rq->curr = next" or via context_switch() updating
    p->oncpu -- which one matters depends on #ifdef __ARCH_WANT_UNLOCKED_CTXSW).

    I haven't been able to find any case where task_running() can be true
    when (p->se.on_rq) is false, while the rq lock is not being held. Thus
    I don't think the branch to out_activate will ever be taken.

    What am I missing, or is the task_running() test not needed?



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