Hugh Dickins wrote:
On Thu, 3 May 2007, Nick Piggin wrote:
The problem is that lock/unlock_page is expensive on powerpc, and
if we improve that, we improve more than just the fault handler...
The attached patch gets performance up a bit by avoiding some
barriers and some cachelines:
There's a strong whiff of raciness about this...
but I could very easily be wrong.
Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c 2007-05-02 15:00:26.000000000 +1000
+++ linux-2.6/mm/filemap.c 2007-05-03 08:34:32.000000000 +1000
@@ -532,11 +532,13 @@
*/
void fastcall unlock_page(struct page *page)
{
+ VM_BUG_ON(!PageLocked(page));
smp_mb__before_clear_bit();
- if (!TestClearPageLocked(page))
- BUG();
- smp_mb__after_clear_bit();
- wake_up_page(page, PG_locked);
+ ClearPageLocked(page);
+ if (unlikely(test_bit(PG_waiters, &page->flags))) {
+ clear_bit(PG_waiters, &page->flags);
+ wake_up_page(page, PG_locked);
+ }
}
EXPORT_SYMBOL(unlock_page);
@@ -568,6 +570,11 @@ __lock_page (diff -p would tell us!)
{
DEFINE_WAIT_BIT(wait, &page->flags, PG_locked);
+ set_bit(PG_waiters, &page->flags);
+ if (unlikely(!TestSetPageLocked(page))) {
What happens if another cpu is coming through __lock_page at the
same time, did its set_bit, now finds PageLocked, and so proceeds
to the __wait_on_bit_lock? But this cpu now clears PG_waiters,
so this task's unlock_page won't wake the other?
You're right, we can't clear the bit here. Doubt it mattered much anyway?
BTW. I also forgot an smp_mb__after_clear_bit() before the wake_up_page
above... that barrier is in the slow path as well though, so it shouldn't
matter either.
+ clear_bit(PG_waiters, &page->flags);
+ return;
+ }
__wait_on_bit_lock(page_waitqueue(page), &wait, sync_page,
TASK_UNINTERRUPTIBLE);
}
--
SUSE Labs, Novell Inc.
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