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    Subject[PATCH 1/9] timer locking optimization

    This simplifies the work done lock_timer_base() and moves it to the less
    common path at __mod_timer(). Instead of setting the base to NULL we
    already set it to the new base there. If another __mod_timer() gets the
    lock it can only move the base to another cpu, so it's enough to check
    that base is still the same.
    Only add_timer_on() could add timer on the current cpu, but this is not
    legal, so we don't have to recheck that the timer has been reactivated
    on the current cpu.

    Signed-off-by: Roman Zippel <zippel@linux-m68k.org>

    ---

    kernel/timer.c | 33 +++++++++++++++++----------------
    1 file changed, 17 insertions(+), 16 deletions(-)

    Index: linux-2.6-mm/kernel/timer.c
    ===================================================================
    --- linux-2.6-mm.orig/kernel/timer.c 2005-11-28 22:32:47.000000000 +0100
    +++ linux-2.6-mm/kernel/timer.c 2005-11-28 23:04:46.000000000 +0100
    @@ -178,27 +178,22 @@ static inline void detach_timer(struct t
    *
    * So __run_timers/migrate_timers can safely modify all timers which could
    * be found on ->tvX lists.
    - *
    - * When the timer's base is locked, and the timer removed from list, it is
    - * possible to set timer->base = NULL and drop the lock: the timer remains
    - * locked.
    */
    static timer_base_t *lock_timer_base(struct timer_list *timer,
    unsigned long *flags)
    {
    timer_base_t *base;

    - for (;;) {
    - base = timer->base;
    - if (likely(base != NULL)) {
    - spin_lock_irqsave(&base->lock, *flags);
    - if (likely(base == timer->base))
    - return base;
    - /* The timer has migrated to another CPU */
    - spin_unlock_irqrestore(&base->lock, *flags);
    - }
    + base = timer->base;
    + spin_lock_irqsave(&base->lock, *flags);
    + while (unlikely(base != timer->base)) {
    + /* The timer has migrated to another CPU */
    + spin_unlock(&base->lock);
    cpu_relax();
    + base = timer->base;
    + spin_lock(&base->lock);
    }
    + return base;
    }

    int __mod_timer(struct timer_list *timer, unsigned long expires)
    @@ -212,6 +207,7 @@ int __mod_timer(struct timer_list *timer

    base = lock_timer_base(timer, &flags);

    +restart:
    if (timer_pending(timer)) {
    detach_timer(timer, 0);
    ret = 1;
    @@ -231,11 +227,16 @@ int __mod_timer(struct timer_list *timer
    /* The timer remains on a former base */
    new_base = container_of(base, tvec_base_t, t_base);
    } else {
    - /* See the comment in lock_timer_base() */
    - timer->base = NULL;
    + /*
    + * We shortly release the timer and the timer can
    + * migrate to another cpu, so recheck the base after
    + * getting the lock.
    + */
    + timer->base = &new_base->t_base;
    spin_unlock(&base->lock);
    spin_lock(&new_base->t_base.lock);
    - timer->base = &new_base->t_base;
    + if (unlikely(timer->base != &new_base->t_base))
    + goto restart;
    }
    }

    -
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      Last update: 2005-11-29 02:38    [W:0.029 / U:58.792 seconds]
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