Re: [REPOST-v2] sched: Prevent wakeup to enter critical section needlessly
From: Oleg Nesterov <oleg@redhat.com>
Date: 2012-11-19 15:10:45
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lkml
On 11/19, Ivo Sieben wrote:
quoted hunk ↗ jump to hunk
--- 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.