mm/slab: refill cpu cache through a new slab without holding a node lock

Until now, cache growing makes a free slab on node's slab list and then
we can allocate free objects from it.  This necessarily requires to hold
a node lock which is very contended.  If we refill cpu cache before
attaching it to node's slab list, we can avoid holding a node lock as
much as possible because this newly allocated slab is only visible to
the current task.  This will reduce lock contention.

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=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

  * After
  Kmalloc N*alloc N*free(32): Average=248/966
  Kmalloc N*alloc N*free(64): Average=261/949
  Kmalloc N*alloc N*free(128): Average=314/1016
  Kmalloc N*alloc N*free(256): Average=741/1061
  Kmalloc N*alloc N*free(512): Average=1246/1152
  Kmalloc N*alloc N*free(1024): Average=2437/1259
  Kmalloc N*alloc N*free(2048): Average=4980/1800
  Kmalloc N*alloc N*free(4096): Average=9000/2078

It shows that contention is reduced for all the object sizes and
performance increases by 30 ~ 40%.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Christoph Lameter <cl@linux.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>

diff --git a/mm/slab.c b/mm/slab.c
index 8c4db21..37600e9 100644 (file)
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2865,6 +2865,30 @@ static noinline void *cache_alloc_pfmemalloc(struct kmem_cache *cachep,
        return obj;
 }
 
+/*
+ * Slab list should be fixed up by fixup_slab_list() for existing slab
+ * or cache_grow_end() for new slab
+ */
+static __always_inline int alloc_block(struct kmem_cache *cachep,
+               struct array_cache *ac, struct page *page, int batchcount)
+{
+       /*
+        * There must be at least one object available for
+        * allocation.
+        */
+       BUG_ON(page->active >= cachep->num);
+
+       while (page->active < cachep->num && batchcount--) {
+               STATS_INC_ALLOCED(cachep);
+               STATS_INC_ACTIVE(cachep);
+               STATS_SET_HIGH(cachep);
+
+               ac->entry[ac->avail++] = slab_get_obj(cachep, page);
+       }
+
+       return batchcount;
+}
+
 static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags)
 {
        int batchcount;
@@ -2877,7 +2901,6 @@ static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags)
        check_irq_off();
        node = numa_mem_id();
 
-retry:
        ac = cpu_cache_get(cachep);
        batchcount = ac->batchcount;
        if (!ac->touched && batchcount > BATCHREFILL_LIMIT) {
@@ -2907,21 +2930,7 @@ retry:
 
                check_spinlock_acquired(cachep);
 
-               /*
-                * The slab was either on partial or free list so
-                * there must be at least one object available for
-                * allocation.
-                */
-               BUG_ON(page->active >= cachep->num);
-
-               while (page->active < cachep->num && batchcount--) {
-                       STATS_INC_ALLOCED(cachep);
-                       STATS_INC_ACTIVE(cachep);
-                       STATS_SET_HIGH(cachep);
-
-                       ac->entry[ac->avail++] = slab_get_obj(cachep, page);
-               }
-
+               batchcount = alloc_block(cachep, ac, page, batchcount);
                fixup_slab_list(cachep, n, page, &list);
        }
 
@@ -2941,21 +2950,18 @@ alloc_done:
                }
 
                page = cache_grow_begin(cachep, gfp_exact_node(flags), node);
-               cache_grow_end(cachep, page);
 
                /*
                 * cache_grow_begin() can reenable interrupts,
                 * then ac could change.
                 */
                ac = cpu_cache_get(cachep);
-               node = numa_mem_id();
+               if (!ac->avail && page)
+                       alloc_block(cachep, ac, page, batchcount);
+               cache_grow_end(cachep, page);
 
-               /* no objects in sight? abort */
-               if (!page && ac->avail == 0)
+               if (!ac->avail)
                        return NULL;
-
-               if (!ac->avail)         /* objects refilled by interrupt? */
-                       goto retry;
        }
        ac->touched = 1;
 
@@ -3149,14 +3155,13 @@ static void *____cache_alloc_node(struct kmem_cache *cachep, gfp_t flags,
 {
        struct page *page;
        struct kmem_cache_node *n;
-       void *obj;
+       void *obj = NULL;
        void *list = NULL;
 
        VM_BUG_ON(nodeid < 0 || nodeid >= MAX_NUMNODES);
        n = get_node(cachep, nodeid);
        BUG_ON(!n);
 
-retry:
        check_irq_off();
        spin_lock(&n->list_lock);
        page = get_first_slab(n, false);
@@ -3178,19 +3183,18 @@ retry:
 
        spin_unlock(&n->list_lock);
        fixup_objfreelist_debug(cachep, &list);
-       goto done;
+       return obj;
 
 must_grow:
        spin_unlock(&n->list_lock);
        page = cache_grow_begin(cachep, gfp_exact_node(flags), nodeid);
+       if (page) {
+               /* This slab isn't counted yet so don't update free_objects */
+               obj = slab_get_obj(cachep, page);
+       }
        cache_grow_end(cachep, page);
-       if (page)
-               goto retry;
 
-       return fallback_alloc(cachep, flags);
-
-done:
-       return obj;
+       return obj ? obj : fallback_alloc(cachep, flags);
 }
 
 static __always_inline void *