On Thu, May 22, 2014 at 01:07:15AM +0100, Mel Gorman wrote:
> +PAGEFLAG(Waiters, waiters) __CLEARPAGEFLAG(Waiters, waiters)
> + TESTCLEARFLAG(Waiters, waiters)
> +#define __PG_WAITERS (1 << PG_waiters)
> +#else
> +/* Always fallback to slow path on 32-bit */
> +static inline bool PageWaiters(struct page *page)
> +{
> + return true;
> +}
> +static inline void __ClearPageWaiters(struct page *page) {}
> +static inline void ClearPageWaiters(struct page *page) {}
> +static inline void SetPageWaiters(struct page *page) {}
> +#define __PG_WAITERS 0
> +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;
> +
> + /*
> + * 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
> + */
Well, you could do something like:
if (!__PG_WAITERS && !waitqueue_active(wqh))
return;
Which at least for 32bit restores some of the performance loss of this
patch (did you have 32bit numbers in that massive changelog?, I totally
tl;dr it).
> + 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);
> +}
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