slab: Bypass free lists for __drain_alien_cache()

[Christoph Lameter <clameter@sgi.com>]

__drain_alien_cache() currently drains objects by freeing them to the 
(remote) freelists of the original node. However, each node also has a 
shared list containing objects to be used on any processor of that node. 
We can avoid a number of remote node accesses by copying the pointers to 
the free objects directly into the remote shared array.

Depends on the earlier patch that introduces the transfer_objects() function.

And while we are at it: Skip alien draining if the alien cache spinlock is
already taken.

Kiran reported that this is a performance benefit.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6.16-rc6-mm2/mm/slab.c
===================================================================
--- linux-2.6.16-rc6-mm2.orig/mm/slab.c	2006-03-21 14:53:40.000000000 -0800
+++ linux-2.6.16-rc6-mm2/mm/slab.c	2006-03-21 14:55:09.000000000 -0800
@@ -974,6 +974,13 @@ static void __drain_alien_cache(struct k
 
 	if (ac->avail) {
 		spin_lock(&rl3->list_lock);
+		/*
+		 * Stuff objects into the remote nodes shared array first.
+		 * That way we could avoid the overhead of putting the objects
+		 * into the free lists and getting them back later.
+		 */
+		transfer_objects(rl3->shared, ac, ac->limit);
+
 		free_block(cachep, ac->entry, ac->avail, node);
 		ac->avail = 0;
 		spin_unlock(&rl3->list_lock);
@@ -989,8 +996,8 @@ static void reap_alien(struct kmem_cache
 
 	if (l3->alien) {
 		struct array_cache *ac = l3->alien[node];
-		if (ac && ac->avail) {
-			spin_lock_irq(&ac->lock);
+
+		if (ac && ac->avail && spin_trylock_irq(&ac->lock)) {
 			__drain_alien_cache(cachep, ac, node);
 			spin_unlock_irq(&ac->lock);
 		}

-
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