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fast_hash: avoid indirect function calls

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By default the arch_fast_hash hashing function pointers are initialized
to jhash(2). If during boot-up a CPU with SSE4.2 is detected they get
updated to the CRC32 ones. This dispatching scheme incurs a function
pointer lookup and indirect call for every hashing operation.

rhashtable as a user of arch_fast_hash e.g. stores pointers to hashing
functions in its structure, too, causing two indirect branches per
hashing operation.

Using alternative_call we can get away with one of those indirect branches.

Acked-by: Daniel Borkmann <[email protected]>
Cc: Thomas Graf <[email protected]>
Signed-off-by: Hannes Frederic Sowa <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
This commit is contained in:
Hannes Frederic Sowa 2014-11-05 00:23:04 +01:00 • committed by David S. Miller
commit e5a2c89995
6 changed files with 98 additions and 93 deletions
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2
lib/Makefile
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@ -26,7 +26,7 @@ obj-y += bcd.o div64.o sort.o parser.o halfmd4.o debug_locks.o random32.o \
bust_spinlocks.o hexdump.o kasprintf.o bitmap.o scatterlist.o \
gcd.o lcm.o list_sort.o uuid.o flex_array.o iovec.o clz_ctz.o \
bsearch.o find_last_bit.o find_next_bit.o llist.o memweight.o kfifo.o \
percpu-refcount.o percpu_ida.o hash.o rhashtable.o
percpu-refcount.o percpu_ida.o rhashtable.o
obj-y += string_helpers.o
obj-$(CONFIG_TEST_STRING_HELPERS) += test-string_helpers.o
obj-y += kstrtox.o

39
lib/hash.c
View file

@ -1,39 +0,0 @@
/* General purpose hashing library
*
* That's a start of a kernel hashing library, which can be extended
* with further algorithms in future. arch_fast_hash{2,}() will
* eventually resolve to an architecture optimized implementation.
*
* Copyright 2013 Francesco Fusco <ffusco@redhat.com>
* Copyright 2013 Daniel Borkmann <dborkman@redhat.com>
* Copyright 2013 Thomas Graf <tgraf@redhat.com>
* Licensed under the GNU General Public License, version 2.0 (GPLv2)
*/
#include <linux/jhash.h>
#include <linux/hash.h>
#include <linux/cache.h>
static struct fast_hash_ops arch_hash_ops __read_mostly = {
.hash = jhash,
.hash2 = jhash2,
};
u32 arch_fast_hash(const void *data, u32 len, u32 seed)
{
return arch_hash_ops.hash(data, len, seed);
}
EXPORT_SYMBOL_GPL(arch_fast_hash);
u32 arch_fast_hash2(const u32 *data, u32 len, u32 seed)
{
return arch_hash_ops.hash2(data, len, seed);
}
EXPORT_SYMBOL_GPL(arch_fast_hash2);
static int __init hashlib_init(void)
{
setup_arch_fast_hash(&arch_hash_ops);
return 0;
}
early_initcall(hashlib_init);