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    Subject[(RT RFC) PATCH v2 4/9] optimize rt lock wakeup
    Date
    It is redundant to wake the grantee task if it is already running

    Credit goes to Peter for the general idea.

    Signed-off-by: Gregory Haskins <ghaskins@novell.com>
    Signed-off-by: Peter Morreale <pmorreale@novell.com>
    ---

    kernel/rtmutex.c | 45 ++++++++++++++++++++++++++++++++++++++++-----
    1 files changed, 40 insertions(+), 5 deletions(-)

    diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
    index ef52db6..bf9e230 100644
    --- a/kernel/rtmutex.c
    +++ b/kernel/rtmutex.c
    @@ -531,6 +531,41 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
    pendowner = waiter->task;
    waiter->task = NULL;

    + /*
    + * Do the wakeup before the ownership change to give any spinning
    + * waiter grantees a headstart over the other threads that will
    + * trigger once owner changes.
    + */
    + if (!savestate)
    + wake_up_process(pendowner);
    + else {
    + /*
    + * We can skip the actual (expensive) wakeup if the
    + * waiter is already running, but we have to be careful
    + * of race conditions because they may be about to sleep.
    + *
    + * The waiter-side protocol has the following pattern:
    + * 1: Set state != RUNNING
    + * 2: Conditionally sleep if waiter->task != NULL;
    + *
    + * And the owner-side has the following:
    + * A: Set waiter->task = NULL
    + * B: Conditionally wake if the state != RUNNING
    + *
    + * As long as we ensure 1->2 order, and A->B order, we
    + * will never miss a wakeup.
    + *
    + * Therefore, this barrier ensures that waiter->task = NULL
    + * is visible before we test the pendowner->state. The
    + * corresponding barrier is in the sleep logic.
    + */
    + smp_mb();
    +
    + if ((pendowner->state != TASK_RUNNING)
    + && (pendowner->state != TASK_RUNNING_MUTEX))
    + wake_up_process_mutex(pendowner);
    + }
    +
    rt_mutex_set_owner(lock, pendowner, RT_MUTEX_OWNER_PENDING);

    spin_unlock(&current->pi_lock);
    @@ -557,11 +592,6 @@ static void wakeup_next_waiter(struct rt_mutex *lock, int savestate)
    plist_add(&next->pi_list_entry, &pendowner->pi_waiters);
    }
    spin_unlock(&pendowner->pi_lock);
    -
    - if (savestate)
    - wake_up_process_mutex(pendowner);
    - else
    - wake_up_process(pendowner);
    }

    /*
    @@ -762,6 +792,11 @@ rt_spin_lock_slowlock(struct rt_mutex *lock)
    debug_rt_mutex_print_deadlock(&waiter);

    update_current(TASK_UNINTERRUPTIBLE, &saved_state);
    + /*
    + * The xchg() in update_current() is an implicit barrier
    + * which we rely upon to ensure current->state is visible
    + * before we test waiter.task.
    + */
    if (waiter.task)
    schedule_rt_mutex(lock);
    else


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