diff options
| -rw-r--r-- | include/linux/reciprocal_div.h | 32 | ||||
| -rw-r--r-- | lib/Makefile | 2 | ||||
| -rw-r--r-- | lib/reciprocal_div.c | 9 | ||||
| -rw-r--r-- | mm/slab.c | 18 |
4 files changed, 57 insertions, 4 deletions
diff --git a/include/linux/reciprocal_div.h b/include/linux/reciprocal_div.h new file mode 100644 index 000000000000..f9c90b33285b --- /dev/null +++ b/include/linux/reciprocal_div.h @@ -0,0 +1,32 @@ +#ifndef _LINUX_RECIPROCAL_DIV_H +#define _LINUX_RECIPROCAL_DIV_H + +#include <linux/types.h> + +/* + * This file describes reciprocical division. + * + * This optimizes the (A/B) problem, when A and B are two u32 + * and B is a known value (but not known at compile time) + * + * The math principle used is : + * Let RECIPROCAL_VALUE(B) be (((1LL << 32) + (B - 1))/ B) + * Then A / B = (u32)(((u64)(A) * (R)) >> 32) + * + * This replaces a divide by a multiply (and a shift), and + * is generally less expensive in CPU cycles. + */ + +/* + * Computes the reciprocal value (R) for the value B of the divisor. + * Should not be called before each reciprocal_divide(), + * or else the performance is slower than a normal divide. + */ +extern u32 reciprocal_value(u32 B); + + +static inline u32 reciprocal_divide(u32 A, u32 R) +{ + return (u32)(((u64)A * R) >> 32); +} +#endif diff --git a/lib/Makefile b/lib/Makefile index 2d6106af53cd..c9ec8f11e833 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -5,7 +5,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \ bust_spinlocks.o rbtree.o radix-tree.o dump_stack.o \ idr.o div64.o int_sqrt.o bitmap.o extable.o prio_tree.o \ - sha1.o irq_regs.o + sha1.o irq_regs.o reciprocal_div.o lib-$(CONFIG_MMU) += ioremap.o lib-$(CONFIG_SMP) += cpumask.o diff --git a/lib/reciprocal_div.c b/lib/reciprocal_div.c new file mode 100644 index 000000000000..6a3bd48fa2a0 --- /dev/null +++ b/lib/reciprocal_div.c @@ -0,0 +1,9 @@ +#include <asm/div64.h> +#include <linux/reciprocal_div.h> + +u32 reciprocal_value(u32 k) +{ + u64 val = (1LL << 32) + (k - 1); + do_div(val, k); + return (u32)val; +} diff --git a/mm/slab.c b/mm/slab.c index b856786a3a30..909975f6e090 100644 --- a/mm/slab.c +++ b/mm/slab.c @@ -109,6 +109,7 @@ #include <linux/mutex.h> #include <linux/fault-inject.h> #include <linux/rtmutex.h> +#include <linux/reciprocal_div.h> #include <asm/cacheflush.h> #include <asm/tlbflush.h> @@ -386,6 +387,7 @@ struct kmem_cache { unsigned int shared; unsigned int buffer_size; + u32 reciprocal_buffer_size; /* 3) touched by every alloc & free from the backend */ struct kmem_list3 *nodelists[MAX_NUMNODES]; @@ -627,10 +629,17 @@ static inline void *index_to_obj(struct kmem_cache *cache, struct slab *slab, return slab->s_mem + cache->buffer_size * idx; } -static inline unsigned int obj_to_index(struct kmem_cache *cache, - struct slab *slab, void *obj) +/* + * We want to avoid an expensive divide : (offset / cache->buffer_size) + * Using the fact that buffer_size is a constant for a particular cache, + * we can replace (offset / cache->buffer_size) by + * reciprocal_divide(offset, cache->reciprocal_buffer_size) + */ +static inline unsigned int obj_to_index(const struct kmem_cache *cache, + const struct slab *slab, void *obj) { - return (unsigned)(obj - slab->s_mem) / cache->buffer_size; + u32 offset = (obj - slab->s_mem); + return reciprocal_divide(offset, cache->reciprocal_buffer_size); } /* @@ -1427,6 +1436,8 @@ void __init kmem_cache_init(void) cache_cache.buffer_size = ALIGN(cache_cache.buffer_size, cache_line_size()); + cache_cache.reciprocal_buffer_size = + reciprocal_value(cache_cache.buffer_size); for (order = 0; order < MAX_ORDER; order++) { cache_estimate(order, cache_cache.buffer_size, @@ -2313,6 +2324,7 @@ kmem_cache_create (const char *name, size_t size, size_t align, if (flags & SLAB_CACHE_DMA) cachep->gfpflags |= GFP_DMA; cachep->buffer_size = size; + cachep->reciprocal_buffer_size = reciprocal_value(size); if (flags & CFLGS_OFF_SLAB) { cachep->slabp_cache = kmem_find_general_cachep(slab_size, 0u); |