linux-kernel - Re: [cpuops cmpxchg V2 5/5] cpuops: Use cmpxchg for xchg to avoid lock semantics

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Date:	Tue, 14 Dec 2010 11:19:24 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	"H. Peter Anvin" <hpa@...or.com>
cc:	Eric Dumazet <eric.dumazet@...il.com>, Tejun Heo <tj@...nel.org>,
	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [cpuops cmpxchg V2 5/5] cpuops: Use cmpxchg for xchg to avoid
 lock semantics



On Tue, 14 Dec 2010, H. Peter Anvin wrote:

> On 12/14/2010 08:55 AM, Christoph Lameter wrote:
> >
> > We do not need to reload the value as pointed out by Eric. It is already in
> > the correct register so just rerun the cmpxchg without the load.
> >
> > Signed-off-by: Christoph Lameter <cl@...ux.com>
> >
>
> Is it genuinely faster to do the pre-load mov, or can we drop that too?
>  My guess would be that yes it is, but if it happens not to be it would
> be nice to reduce the code size.

Dropping the load increases the cycle count from 11 to 16.

---
 arch/x86/include/asm/percpu.h |   12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

Index: linux-2.6/arch/x86/include/asm/percpu.h
===================================================================
--- linux-2.6.orig/arch/x86/include/asm/percpu.h	2010-12-14 11:07:18.000000000 -0600
+++ linux-2.6/arch/x86/include/asm/percpu.h	2010-12-14 11:14:37.000000000 -0600
@@ -223,32 +223,28 @@ do {									\
 	typeof(var) __new = (nval);					\
 	switch (sizeof(var)) {						\
 	case 1:								\
-		asm("\n1:mov "__percpu_arg(1)",%%al"			\
-		    "\n\tcmpxchgb %2, "__percpu_arg(1)			\
+		asm("\n1:cmpxchgb %2, "__percpu_arg(1)			\
 		    "\n\tjnz 1b"					\
 			    : "=a" (__ret), "+m" (var)			\
 			    : "q" (__new)				\
 			    : "memory");				\
 		break;							\
 	case 2:								\
-		asm("\n1:mov "__percpu_arg(1)",%%ax"			\
-		    "\n\tcmpxchgw %2, "__percpu_arg(1)			\
+		asm("\n1:cmpxchgw %2, "__percpu_arg(1)			\
 		    "\n\tjnz 1b"					\
 			    : "=a" (__ret), "+m" (var)			\
 			    : "r" (__new)				\
 			    : "memory");				\
 		break;							\
 	case 4:								\
-		asm("\n1:mov "__percpu_arg(1)",%%eax"			\
-		    "\n\tcmpxchgl %2, "__percpu_arg(1)			\
+		asm("\n1:cmpxchgl %2, "__percpu_arg(1)			\
 		    "\n\tjnz 1b"					\
 			    : "=a" (__ret), "+m" (var)			\
 			    : "r" (__new)				\
 			    : "memory");				\
 		break;							\
 	case 8:								\
-		asm("\n1:mov "__percpu_arg(1)",%%rax"			\
-		    "\n\tcmpxchgq %2, "__percpu_arg(1)			\
+		asm("\n1:cmpxchgq %2, "__percpu_arg(1)			\
 		    "\n\tjnz 1b"					\
 			    : "=a" (__ret), "+m" (var)			\
 			    : "r" (__new)				\
--
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