From: Joonsoo Kim Date: Fri, 20 May 2016 00:10:31 +0000 (-0700) Subject: mm/slab: lockless decision to grow cache X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=801faf0db894;p=linux-beck.git mm/slab: lockless decision to grow cache To check whether 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 Cc: Jesper Dangaard Brouer Cc: Christoph Lameter Cc: Pekka Enberg Cc: David Rientjes Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/slab.c b/mm/slab.c index 37600e91742f..8133ebea77a4 100644 --- a/mm/slab.c +++ b/mm/slab.c @@ -965,6 +965,15 @@ 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 synchronize_sched(). + */ + if (force_change) + synchronize_sched(); + fail: kfree(old_shared); kfree(new_shared); @@ -2893,7 +2902,7 @@ 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; @@ -2914,11 +2923,16 @@ 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; } @@ -2940,6 +2954,7 @@ 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()) {