[PATCH 11/11] mm/slab: lockless decision to grow cache

[js1304]

From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

To check whther free objects exist or not precisely, we need to grab
a lock. But, accuracy isn't that important because race window would
be even small and if there is too much free object, cache reaper would
reap it. So, this patch makes the check for free object exisistence
not to hold a lock. This will reduce lock contention in heavily
allocation case.

Note that until now, n->shared can be freed during the processing by
writing slabinfo, but, with some trick in this patch, we can access it
freely within interrupt disabled period.

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=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

* After
Kmalloc N*alloc N*free(32): Average=344/792
Kmalloc N*alloc N*free(64): Average=347/882
Kmalloc N*alloc N*free(128): Average=390/959
Kmalloc N*alloc N*free(256): Average=393/1067
Kmalloc N*alloc N*free(512): Average=683/1229
Kmalloc N*alloc N*free(1024): Average=1295/1325
Kmalloc N*alloc N*free(2048): Average=2513/1664
Kmalloc N*alloc N*free(4096): Average=4742/2172

It shows that allocation performance decreases for the object size
up to 128 and it may be due to extra checks in cache_alloc_refill().
But, with considering improvement of free performance, net result looks
the same. Result for other size class looks very promising, roughly,
50% performance improvement.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
---
 mm/slab.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 029d6b3..b70aabf 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -951,6 +951,15 @@ static int setup_kmem_cache_node(struct kmem_cache *cachep,
 	spin_unlock_irq(&n->list_lock);
 	slabs_destroy(cachep, &list);
 
+	/*
+	 * To protect lockless access to n->shared during irq disabled context.
+	 * If n->shared isn't NULL in irq disabled context, accessing to it is
+	 * guaranteed to be valid until irq is re-enabled, because it will be
+	 * freed after kick_all_cpus_sync().
+	 */
+	if (force_change)
+		kick_all_cpus_sync();
+
 fail:
 	kfree(old_shared);
 	kfree(new_shared);
@@ -2855,7 +2864,7 @@ static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags)
 {
 	int batchcount;
 	struct kmem_cache_node *n;
-	struct array_cache *ac;
+	struct array_cache *ac, *shared;
 	int node;
 	void *list = NULL;
 	struct page *page;
@@ -2876,11 +2885,16 @@ static void *cache_alloc_refill(struct kmem_cache *cachep, gfp_t flags)
 	n = get_node(cachep, node);
 
 	BUG_ON(ac->avail > 0 || !n);
+	shared = READ_ONCE(n->shared);
+	if (!n->free_objects && (!shared || !shared->avail))
+		goto direct_grow;
+
 	spin_lock(&n->list_lock);
+	shared = READ_ONCE(n->shared);
 
 	/* See if we can refill from the shared array */
-	if (n->shared && transfer_objects(ac, n->shared, batchcount)) {
-		n->shared->touched = 1;
+	if (shared && transfer_objects(ac, shared, batchcount)) {
+		shared->touched = 1;
 		goto alloc_done;
 	}
 
@@ -2902,6 +2916,7 @@ alloc_done:
 	spin_unlock(&n->list_lock);
 	fixup_objfreelist_debug(cachep, &list);
 
+direct_grow:
 	if (unlikely(!ac->avail)) {
 		/* Check if we can use obj in pfmemalloc slab */
 		if (sk_memalloc_socks()) {
-- 
1.9.1