[PATCH 07/11] mm/slab: racy access/modify the slab color

[js1304]

From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Slab color isn't needed to be changed strictly. Because locking
for changing slab color could cause more lock contention so this patch
implements racy access/modify the slab color. This is a preparation step
to implement lockless allocation path when there is no free objects in
the kmem_cache.

Below is the result of concurrent allocation/free in slab allocation
benchmark made by Christoph a long time ago. I make the output simpler.
The number shows cycle count during alloc/free respectively so less
is better.

* Before
Kmalloc N*alloc N*free(32): Average=365/806
Kmalloc N*alloc N*free(64): Average=452/690
Kmalloc N*alloc N*free(128): Average=736/886
Kmalloc N*alloc N*free(256): Average=1167/985
Kmalloc N*alloc N*free(512): Average=2088/1125
Kmalloc N*alloc N*free(1024): Average=4115/1184
Kmalloc N*alloc N*free(2048): Average=8451/1748
Kmalloc N*alloc N*free(4096): Average=16024/2048

* After
Kmalloc N*alloc N*free(32): Average=355/750
Kmalloc N*alloc N*free(64): Average=452/812
Kmalloc N*alloc N*free(128): Average=559/1070
Kmalloc N*alloc N*free(256): Average=1176/980
Kmalloc N*alloc N*free(512): Average=1939/1189
Kmalloc N*alloc N*free(1024): Average=3521/1278
Kmalloc N*alloc N*free(2048): Average=7152/1838
Kmalloc N*alloc N*free(4096): Average=13438/2013

It shows that contention is reduced for object size >= 1024
and performance increases by roughly 15%.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
 mm/slab.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index df11757..52fc5e3 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2536,20 +2536,7 @@ static int cache_grow(struct kmem_cache *cachep,
 	}
 	local_flags = flags & (GFP_CONSTRAINT_MASK|GFP_RECLAIM_MASK);
 
-	/* Take the node list lock to change the colour_next on this node */
 	check_irq_off();
-	n = get_node(cachep, nodeid);
-	spin_lock(&n->list_lock);
-
-	/* Get colour for the slab, and cal the next value. */
-	offset = n->colour_next;
-	n->colour_next++;
-	if (n->colour_next >= cachep->colour)
-		n->colour_next = 0;
-	spin_unlock(&n->list_lock);
-
-	offset *= cachep->colour_off;
-
 	if (gfpflags_allow_blocking(local_flags))
 		local_irq_enable();
 
@@ -2570,6 +2557,19 @@ static int cache_grow(struct kmem_cache *cachep,
 	if (!page)
 		goto failed;
 
+	n = get_node(cachep, nodeid);
+
+	/* Get colour for the slab, and cal the next value. */
+	n->colour_next++;
+	if (n->colour_next >= cachep->colour)
+		n->colour_next = 0;
+
+	offset = n->colour_next;
+	if (offset >= cachep->colour)
+		offset = 0;
+
+	offset *= cachep->colour_off;
+
 	/* Get slab management. */
 	freelist = alloc_slabmgmt(cachep, page, offset,
 			local_flags & ~GFP_CONSTRAINT_MASK, nodeid);
-- 
1.9.1