mm/slab: racy access/modify the slab color

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@lge.com>
Acked-by: Christoph Lameter <cl@linux.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.c

@@ -2574,20 +2574,7 @@ 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();
 
@@ -2608,6 +2595,19 @@ 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);