Re: [cpuops cmpxchg V2 5/5] cpuops: Use cmpxchg for xchg to avoidlock semantics

[Eric Dumazet]

Le mardi 14 dÃcembre 2010 Ã 10:28 -0600, Christoph Lameter a Ãcrit :
> piÃce jointe document texte brut (cpuops_xchg_with_cmpxchg)
> 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@xxxxxxxxx>
> 
> ---
>  arch/x86/include/asm/percpu.h |   21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> Index: linux-2.6/arch/x86/include/asm/percpu.h
> ===================================================================
> --- linux-2.6.orig/arch/x86/include/asm/percpu.h	2010-12-10 12:46:31.000000000 -0600
> +++ linux-2.6/arch/x86/include/asm/percpu.h	2010-12-10 13:25:21.000000000 -0600
> @@ -213,8 +213,9 @@ 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)					\
>  ({									\
> @@ -222,25 +223,33 @@ do {									\
>  	typeof(var) __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"					\


You should use the fact that the failed cmpxchg loads in al/ax/eax/rax
the current value, so :

	"\n\tmov "__percpu_arg(1)",%%al"
	"\n1:\tcmpxchgb %2, "__percpu_arg(1)
	"\n\tjnz 1b"

(No need to reload the value again)


>  			    : "=a" (__ret), "+m" (var)			\
>  			    : "q" (__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" (__ret), "+m" (var)			\
>  			    : "r" (__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" (__ret), "+m" (var)			\
>  			    : "r" (__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" (__ret), "+m" (var)			\
>  			    : "r" (__new)				\
>  			    : "memory");				\
> 


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