On 11/19, Ivo Sieben wrote:
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3090,9 +3090,22 @@ void __wake_up(wait_queue_head_t *q, unsigned int mode,
> {
> unsigned long flags;
>
> - spin_lock_irqsave(&q->lock, flags);
> - __wake_up_common(q, mode, nr_exclusive, 0, key);
> - spin_unlock_irqrestore(&q->lock, flags);
> + /*
> + * We check for list emptiness outside the lock. This prevents the wake
> + * up to enter the critical section needlessly when the task list is
> + * empty.
> + *
> + * Placed a full memory barrier before checking list emptiness to make
> + * 100% sure this function sees an up-to-date list administration.
> + * Note that other code that manipulates the list uses a spin_lock and
> + * therefore doesn't need additional memory barriers.
> + */
> + smp_mb();
> + if (!list_empty(&q->task_list)) {
waitqueue_active() ?
> + spin_lock_irqsave(&q->lock, flags);
> + __wake_up_common(q, mode, nr_exclusive, 0, key);
> + spin_unlock_irqrestore(&q->lock, flags);
> + }
I am wondering if it makes sense unconditionally. A lot of callers do
if (waitqueue_active(q))
wake_up(...);
this patch makes the optimization above pointless and adds mb().
But I won't argue.
Oleg.
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