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cpuops: Use cmpxchg for xchg to avoid lock semantics 

Use cmpxchg instead of xchg to realize this_cpu_xchg.

xchg will cause LOCK overhead since LOCK is always implied but cmpxchg
will not.

Baselines:

xchg()		= 18 cycles (no segment prefix, LOCK semantics)
__this_cpu_xchg = 1 cycle

(simulated using this_cpu_read/write, two prefixes. Looks like the
cpu can use loop optimization to get rid of most of the overhead)

Cycles before:

this_cpu_xchg	 = 37 cycles (segment prefix and LOCK (implied by xchg))

After:

this_cpu_xchg	= 11 cycle (using cmpxchg without lock semantics)

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Christoph Lameter 2010-12-14 10:28:47 -06:00 committed by Tejun Heo
parent 7296e08aba
commit 8270137a0d
1 changed files with 15 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

21
arch/x86/include/asm/percpu.h
View File

@ -263,8 +263,9 @@ do { \
})
/*
* Beware: xchg on x86 has an implied lock prefix. There will be the cost of
* full lock semantics even though they are not needed.
* xchg is implemented using cmpxchg without a lock prefix. xchg is
* expensive due to the implied lock prefix. The processor cannot prefetch
* cachelines if xchg is used.
*/
#define percpu_xchg_op(var, nval) \
({ \
@ -272,25 +273,33 @@ do { \
typeof(var) pxo_new__ = (nval); \
switch (sizeof(var)) { \
case 1: \
asm("xchgb %2, "__percpu_arg(1) \
asm("\n1:mov "__percpu_arg(1)",%%al" \
"\n\tcmpxchgb %2, "__percpu_arg(1) \
"\n\tjnz 1b" \
: "=a" (pxo_ret__), "+m" (var) \
: "q" (pxo_new__) \
: "memory"); \
break; \
case 2: \
asm("xchgw %2, "__percpu_arg(1) \
asm("\n1:mov "__percpu_arg(1)",%%ax" \
"\n\tcmpxchgw %2, "__percpu_arg(1) \
"\n\tjnz 1b" \
: "=a" (pxo_ret__), "+m" (var) \
: "r" (pxo_new__) \
: "memory"); \
break; \
case 4: \
asm("xchgl %2, "__percpu_arg(1) \
asm("\n1:mov "__percpu_arg(1)",%%eax" \
"\n\tcmpxchgl %2, "__percpu_arg(1) \
"\n\tjnz 1b" \
: "=a" (pxo_ret__), "+m" (var) \
: "r" (pxo_new__) \
: "memory"); \
break; \
case 8: \
asm("xchgq %2, "__percpu_arg(1) \
asm("\n1:mov "__percpu_arg(1)",%%rax" \
"\n\tcmpxchgq %2, "__percpu_arg(1) \
"\n\tjnz 1b" \
: "=a" (pxo_ret__), "+m" (var) \
: "r" (pxo_new__) \
: "memory"); \