On Thu, May 22, 2014 at 11:40:51AM +0100, Mel Gorman wrote:
> +void __wake_up_page_bit(wait_queue_head_t *wqh, struct page *page, void *word, int bit)
> +{
> + struct wait_bit_key key = __WAIT_BIT_KEY_INITIALIZER(word, bit);
> + unsigned long flags;
> +
> + /* If there is no PG_waiters bit, always take the slow path */
That comment is misleading, this is actually a fast path for
!PG_waiters.
> + if (!__PG_WAITERS && waitqueue_active(wq)) {
> + __wake_up(wq, TASK_NORMAL, 1, &key);
> + return;
> + }
> +
> + /*
> + * Unlike __wake_up_bit it is necessary to check waitqueue_active to be
> + * checked under the wqh->lock to avoid races with parallel additions
> + * to the waitqueue. Otherwise races could result in lost wakeups
> + */
> + spin_lock_irqsave(&wqh->lock, flags);
> + if (waitqueue_active(wqh))
> + __wake_up_common(wqh, TASK_NORMAL, 1, 0, &key);
> + else
> + ClearPageWaiters(page);
> + spin_unlock_irqrestore(&wqh->lock, flags);
> +}
So I think you missed one Clear opportunity here that was in my original
proposal, possibly because you also frobbed PG_writeback in.
If you do:
spin_lock_irqsave(&wqh->lock, flags);
if (!waitqueue_active(wqh) || !__wake_up_common(wqh, TASK_NORMAL, 1, 0, &key))
ClearPageWaiters(page);
spin_unlock_irqrestore(&wqh->lock, flags);
With the below change to __wake_up_common(), we'll also clear the bit
when there's no waiters of @page, even if there's waiters for another
page.
I suppose the one thing to say for the big open coded loop is that its
much easier to read than this scattered stuff.
---
diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index 0ffa20ae657b..213c5bfe6b56 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -61,18 +61,23 @@ EXPORT_SYMBOL(remove_wait_queue);
* started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
* zero in this (rare) case, and we handle it by continuing to scan the queue.
*/
-static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
+static bool __wake_up_common(wait_queue_head_t *q, unsigned int mode,
int nr_exclusive, int wake_flags, void *key)
{
wait_queue_t *curr, *next;
+ bool woke = false;
list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
unsigned flags = curr->flags;
- if (curr->func(curr, mode, wake_flags, key) &&
- (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
- break;
+ if (curr->func(curr, mode, wake_flags, key)) {
+ woke = true;
+ if ((flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
+ break;
+ }
}
+
+ return woke;
}
/**
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