Re: [REPOST-v2] sched: Prevent wakeup to enter critical section needlessly

[Oleg Nesterov <oleg@redhat.com>]

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.