Re: [PATCH] mm-slab: allocate kmem_cache with __GFP_REPEAT

Eric Dumazet
2011-07-20T17:04:36Z

Le mercredi 20 juillet 2011 A 18:31 +0200, Eric Dumazet a A(C)crit :

> In fact resulting code is smaller, because most fields are now with <
> 127 offset (x86 assembly code can use shorter instructions)
>
> Before patch :
> # size mm/slab.o
> text data bss dec hex filename
> 22605 361665 32 384302 5dd2e mm/slab.o
>
> After patch :
> # size mm/slab.o
> text data bss dec hex filename
> 22347 328929 32800 384076 5dc4c mm/slab.o
>
> > The per node pointers are lower priority in terms of performance than the
> > per cpu pointers. I'd rather have the per node pointers requiring an
> > additional lookup. Less impact on hot code paths.
> >
>
> Sure. I'll post a V2 to have CPU array before NODE array.
>

Here it is. My dev machine still run after booting with this patch.
No performance impact seen yet.

Thanks

[PATCH v2] slab: shrinks sizeof(struct kmem_cache)

Reduce high order allocations for some setups.
(NR_CPUS=4096 -> we need 64KB per kmem_cache struct)

We now allocate exact needed size (using nr_cpu_ids and nr_node_ids)

This also makes code a bit smaller on x86_64, since some field offsets
are less than the 127 limit :

Before patch :
# size mm/slab.o
text data bss dec hex filename
22605 361665 32 384302 5dd2e mm/slab.o

After patch :
# size mm/slab.o
text data bss dec hex filename
22349 353473 8224 384046 5dc2e mm/slab.o

Reported-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Pekka Enberg <penberg@kernel.org>
CC: Christoph Lameter <cl@linux.com>
CC: Andrew Morton <akpm@linux-foundation.org>
---
v2: move the CPU array before NODE array.

include/linux/slab_def.h | 26 +++++++++++++-------------
mm/slab.c | 10 ++++++----
2 files changed, 19 insertions(+), 17 deletions(-)

diff --git a/include/linux/slab_def.h b/include/linux/slab_def.h
index 83203ae..6e51c4d 100644
--- a/include/linux/slab_def.h
+++ b/include/linux/slab_def.h
@@ -50,21 +50,19 @@
*/

struct kmem_cache {
-/* 1) per-cpu data, touched during every alloc/free */
- struct array_cache *array[NR_CPUS];
-/* 2) Cache tunables. Protected by cache_chain_mutex */
+/* 1) Cache tunables. Protected by cache_chain_mutex */
unsigned int batchcount;
unsigned int limit;
unsigned int shared;

unsigned int buffer_size;
u32 reciprocal_buffer_size;
-/* 3) touched by every alloc & free from the backend */
+/* 2) touched by every alloc & free from the backend */

unsigned int flags; /* constant flags */
unsigned int num; /* # of objs per slab */

-/* 4) cache_grow/shrink */
+/* 3) cache_grow/shrink */
/* order of pgs per slab (2^n) */
unsigned int gfporder;

@@ -80,11 +78,11 @@ struct kmem_cache {
/* constructor func */
void (*ctor)(void *obj);

-/* 5) cache creation/removal */
+/* 4) cache creation/removal */
const char *name;
struct list_head next;

-/* 6) statistics */
+/* 5) statistics */
#ifdef CONFIG_DEBUG_SLAB
unsigned long num_active;
unsigned long num_allocations;
@@ -111,16 +109,18 @@ struct kmem_cache {
int obj_size;
#endif /* CONFIG_DEBUG_SLAB */

+/* 6) per-cpu/per-node data, touched during every alloc/free */
/*
- * We put nodelists[] at the end of kmem_cache, because we want to size
- * this array to nr_node_ids slots instead of MAX_NUMNODES
+ * We put array[] at the end of kmem_cache, because we want to size
+ * this array to nr_cpu_ids slots instead of NR_CPUS
* (see kmem_cache_init())
- * We still use [MAX_NUMNODES] and not [1] or [0] because cache_cache
- * is statically defined, so we reserve the max number of nodes.
+ * We still use [NR_CPUS] and not [1] or [0] because cache_cache
+ * is statically defined, so we reserve the max number of cpus.
*/
- struct kmem_list3 *nodelists[MAX_NUMNODES];
+ struct kmem_list3 **nodelists;
+ struct array_cache *array[NR_CPUS];
/*
- * Do not add fields after nodelists[]
+ * Do not add fields after array[]
*/
};

diff --git a/mm/slab.c b/mm/slab.c
index d96e223..30181fb 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -574,7 +574,9 @@ static struct arraycache_init initarray_generic =
{ {0, BOOT_CPUCACHE_ENTRIES, 1, 0} };

/* internal cache of cache description objs */
+static struct kmem_list3 *cache_cache_nodelists[MAX_NUMNODES];
static struct kmem_cache cache_cache = {
+ .nodelists = cache_cache_nodelists,
.batchcount = 1,
.limit = BOOT_CPUCACHE_ENTRIES,
.shared = 1,
@@ -1492,11 +1494,10 @@ void __init kmem_cache_init(void)
cache_cache.nodelists[node] = &initkmem_list3[CACHE_CACHE + node];

/*
- * struct kmem_cache size depends on nr_node_ids, which
- * can be less than MAX_NUMNODES.
+ * struct kmem_cache size depends on nr_node_ids & nr_cpu_ids
*/
- cache_cache.buffer_size = offsetof(struct kmem_cache, nodelists) +
- nr_node_ids * sizeof(struct kmem_list3 *);
+ cache_cache.buffer_size = offsetof(struct kmem_cache, array[nr_cpu_ids]) +
+ nr_node_ids * sizeof(struct kmem_list3 *);
#if DEBUG
cache_cache.obj_size = cache_cache.buffer_size;
#endif
@@ -2308,6 +2309,7 @@ kmem_cache_create (const char *name, size_t size, size_t align,
if (!cachep)
goto oops;

+ cachep->nodelists = (struct kmem_list3 **)&cachep->array[nr_cpu_ids];
#if DEBUG
cachep->obj_size = size;