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Subject[PATCH 6/6] cpuops: Use cmpxchg for xchg to avoid lock semantics
Date
From: Christoph Lameter <cl@linux.com>

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>
---
arch/x86/include/asm/percpu.h | 21 +++++++++++++++------
1 files changed, 15 insertions(+), 6 deletions(-)
diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index b85ade5..8ee4516 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -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"); \
--
1.7.1


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