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Subject[PATCH] slab: NUMA kmem_cache diet
Some NUMA machines have a big MAX_NUMNODES (possibly 1024), but fewer possible 
nodes. This patch dynamically sizes the 'struct kmem_cache' to allocate only
needed space.

I moved nodelists[] field at the end of struct kmem_cache, and use the
following computation in kmem_cache_init()

cache_cache.buffer_size = offsetof(struct kmem_cache, nodelists) +
nr_node_ids * sizeof(struct kmem_list3 *);


On my two nodes x86_64 machine, kmem_cache.obj_size is now 192 instead of 704
(This is because on x86_64, MAX_NUMNODES is 64)

On bigger NUMA setups, this might reduce the gfporder of "cache_cache"

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>

diff --git a/mm/slab.c b/mm/slab.c
index abf46ae..b187618 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -389,7 +389,6 @@ struct kmem_cache {
unsigned int buffer_size;
u32 reciprocal_buffer_size;
/* 3) touched by every alloc & free from the backend */
- struct kmem_list3 *nodelists[MAX_NUMNODES];

unsigned int flags; /* constant flags */
unsigned int num; /* # of objs per slab */
@@ -444,6 +443,17 @@ #if DEBUG
int obj_offset;
int obj_size;
#endif
+ /*
+ * 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
+ * (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.
+ */
+ struct kmem_list3 *nodelists[MAX_NUMNODES];
+ /*
+ * Do not add fields after nodelists[]
+ */
};

#define CFLGS_OFF_SLAB (0x80000000UL)
@@ -678,9 +688,6 @@ static struct kmem_cache cache_cache = {
.shared = 1,
.buffer_size = sizeof(struct kmem_cache),
.name = "kmem_cache",
-#if DEBUG
- .obj_size = sizeof(struct kmem_cache),
-#endif
};

#define BAD_ALIEN_MAGIC 0x01020304ul
@@ -1437,6 +1444,15 @@ void __init kmem_cache_init(void)
cache_cache.array[smp_processor_id()] = &initarray_cache.cache;
cache_cache.nodelists[node] = &initkmem_list3[CACHE_CACHE];

+ /*
+ * struct kmem_cache size depends on nr_node_ids, which
+ * can be less than MAX_NUMNODES.
+ */
+ cache_cache.buffer_size = offsetof(struct kmem_cache, nodelists) +
+ nr_node_ids * sizeof(struct kmem_list3 *);
+#if DEBUG
+ cache_cache.obj_size = cache_cache.buffer_size;
+#endif
cache_cache.buffer_size = ALIGN(cache_cache.buffer_size,
cache_line_size());
cache_cache.reciprocal_buffer_size =
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