[RFC/PATCH] slub: Drop slab lock for partial list handling

[Pekka J Enberg]

From: Pekka Enberg <penberg@xxxxxxxxxx>

There's no need to hold 'page' slab lock for partial list handling functions. A
page is bound to a node so 'page->lru' is always protected by n->list_lock.

Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Pekka Enberg <penberg@xxxxxxxxxx>
---
David, assuming this actually works, does it help with netperf TCP_RR? The 
patch is on top of my slab/next branch.

 mm/slub.c |   26 +++++++++++++++-----------
 1 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 8fd5401..f41126f 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1452,8 +1452,11 @@ static void unfreeze_slab(struct kmem_cache *s, struct page *page, int tail)

 	__ClearPageSlubFrozen(page);
 	if (page->inuse) {
+		void *prior = page->freelist;

-		if (page->freelist) {
+		slab_unlock(page);
+
+		if (prior) {
 			add_partial(n, page, tail);
 			stat(s, tail ? DEACTIVATE_TO_TAIL : DEACTIVATE_TO_HEAD);
 		} else {
@@ -1461,8 +1464,8 @@ static void unfreeze_slab(struct kmem_cache *s, struct page *page, int tail)
 			if (kmem_cache_debug(s) && (s->flags & SLAB_STORE_USER))
 				add_full(n, page);
 		}
-		slab_unlock(page);
 	} else {
+		slab_unlock(page);
 		stat(s, DEACTIVATE_EMPTY);
 		if (n->nr_partial < s->min_partial) {
 			/*
@@ -1476,9 +1479,7 @@ static void unfreeze_slab(struct kmem_cache *s, struct page *page, int tail)
 			 * the partial list.
 			 */
 			add_partial(n, page, 1);
-			slab_unlock(page);
 		} else {
-			slab_unlock(page);
 			stat(s, FREE_SLAB);
 			discard_slab(s, page);
 		}
@@ -1831,13 +1832,16 @@ checks_ok:
 	page->inuse--;

 	if (unlikely(PageSlubFrozen(page))) {
+		slab_unlock(page);
 		stat(s, FREE_FROZEN);
-		goto out_unlock;
+		goto out;
 	}

 	if (unlikely(!page->inuse))
 		goto slab_empty;

+	slab_unlock(page);
+
 	/*
 	 * Objects left in the slab. If it was not on the partial list before
 	 * then add it.
@@ -1846,12 +1850,11 @@ checks_ok:
 		add_partial(get_node(s, page_to_nid(page)), page, 1);
 		stat(s, FREE_ADD_PARTIAL);
 	}
-
-out_unlock:
-	slab_unlock(page);
+out:
 	return;

 slab_empty:
+	slab_unlock(page);
 	if (prior) {
 		/*
 		 * Slab still on the partial list.
@@ -1859,14 +1862,15 @@ slab_empty:
 		remove_partial(s, page);
 		stat(s, FREE_REMOVE_PARTIAL);
 	}
-	slab_unlock(page);
 	stat(s, FREE_SLAB);
 	discard_slab(s, page);
 	return;

 debug:
-	if (!free_debug_processing(s, page, x, addr))
-		goto out_unlock;
+	if (!free_debug_processing(s, page, x, addr)) {
+		slab_unlock(page);
+		goto out;
+	}
 	goto checks_ok;
 }

--
1.7.1

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