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| author | Avi Kivity <avi@redhat.com> | 2010-02-13 10:33:12 +0200 |
|---|---|---|
| committer | H. Peter Anvin <hpa@zytor.com> | 2010-02-13 13:37:56 -0800 |
| commit | 0d1622d7f526311d87d7da2ee7dd14b73e45d3fc (patch) | |
| tree | eb97e7b70d96faabbbd32cfea8fa34ac5e12eef5 | |
| parent | 1838ef1d782f7527e6defe87e180598622d2d071 (diff) | |
| download | linux-0d1622d7f526311d87d7da2ee7dd14b73e45d3fc.tar.gz linux-0d1622d7f526311d87d7da2ee7dd14b73e45d3fc.tar.bz2 linux-0d1622d7f526311d87d7da2ee7dd14b73e45d3fc.zip | |
x86-64, rwsem: Avoid store forwarding hazard in __downgrade_write
The Intel Architecture Optimization Reference Manual states that a short
load that follows a long store to the same object will suffer a store
forwading penalty, particularly if the two accesses use different addresses.
Trivially, a long load that follows a short store will also suffer a penalty.
__downgrade_write() in rwsem incurs both penalties: the increment operation
will not be able to reuse a recently-loaded rwsem value, and its result will
not be reused by any recently-following rwsem operation.
A comment in the code states that this is because 64-bit immediates are
special and expensive; but while they are slightly special (only a single
instruction allows them), they aren't expensive: a test shows that two loops,
one loading a 32-bit immediate and one loading a 64-bit immediate, both take
1.5 cycles per iteration.
Fix this by changing __downgrade_write to use the same add instruction on
i386 and on x86_64, so that it uses the same operand size as all the other
rwsem functions.
Signed-off-by: Avi Kivity <avi@redhat.com>
LKML-Reference: <1266049992-17419-1-git-send-email-avi@redhat.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
| -rw-r--r-- | arch/x86/include/asm/rwsem.h | 25 |
1 files changed, 5 insertions, 20 deletions
diff --git a/arch/x86/include/asm/rwsem.h b/arch/x86/include/asm/rwsem.h index 10204a25bf93..606ede126972 100644 --- a/arch/x86/include/asm/rwsem.h +++ b/arch/x86/include/asm/rwsem.h @@ -232,34 +232,19 @@ static inline void __up_write(struct rw_semaphore *sem) */ static inline void __downgrade_write(struct rw_semaphore *sem) { -#ifdef CONFIG_X86_64 -# if RWSEM_WAITING_BIAS != -0x100000000 -# error "This code assumes RWSEM_WAITING_BIAS == -2^32" -# endif - - /* 64-bit immediates are special and expensive, and not needed here */ - asm volatile("# beginning __downgrade_write\n\t" - LOCK_PREFIX "incl 4(%1)\n\t" - /* transitions 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 */ - " jns 1f\n\t" - " call call_rwsem_downgrade_wake\n" - "1:\n\t" - "# ending __downgrade_write\n" - : "+m" (sem->count) - : "a" (sem) - : "memory", "cc"); -#else asm volatile("# beginning __downgrade_write\n\t" LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t" - /* transitions 0xZZZZ0001 -> 0xYYYY0001 */ + /* + * transitions 0xZZZZ0001 -> 0xYYYY0001 (i386) + * 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64) + */ " jns 1f\n\t" " call call_rwsem_downgrade_wake\n" "1:\n\t" "# ending __downgrade_write\n" : "+m" (sem->count) - : "a" (sem), "i" (-RWSEM_WAITING_BIAS) + : "a" (sem), "er" (-RWSEM_WAITING_BIAS) : "memory", "cc"); -#endif } /* |