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Date: Tue, 21 Aug 2007 04:21:20 +0200
From: Nick Piggin <npiggin@suse.de>
To: linuxppc-dev@ozlabs.org
Subject: Re: [patch 1/2] powerpc: smp_wmb speedup
Message-ID: <20070821022119.GD2909@wotan.suse.de>
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Cc: Paul Mackerras <paulus@samba.org>
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Sorry, this is patch 2/2 of course.

On Tue, Aug 21, 2007 at 04:16:52AM +0200, Nick Piggin wrote:
> This one is perhaps not as straightforward. I'm pretty limited in the types
> of powerpc machines I can test with, so I don't actually know whether this
> is the right thing to do on power5/6 etc. I can supply the simple test program
> I used if anybody is interested.
> 
> ---
> On my dual G5, lwsync is over 5 times faster than eieio when used in a simple
> test case (that actually makes real use of lwsync to provide write ordering).
> 
> This is not surprising, as it avoids the IO access synchronisation of eieio,
> and still permits the important relaxation of executing loads before stores.
> The on sub-architectures where lwsync is unavailable, eieio is retained, as
> it should be faster than the alternative full sync (eieio is a proper subset
> of sync).
> 
> Signed-off-by: Nick Piggin <npiggin@suse.de>
> 
> Index: linux-2.6/include/asm-powerpc/system.h
> ===================================================================
> --- linux-2.6.orig/include/asm-powerpc/system.h
> +++ linux-2.6/include/asm-powerpc/system.h
> @@ -43,7 +43,11 @@
>  #ifdef CONFIG_SMP
>  #define smp_mb()	mb()
>  #define smp_rmb()	__asm__ __volatile__ (__stringify(LWSYNC) : : : "memory")
> +#ifdef __SUBARCH_HAS_LWSYNC
> +#define smp_wmb()	__asm__ __volatile__ (__stringify(LWSYNC) : : : "memory")
> +#else
>  #define smp_wmb()	eieio()
> +#endif
>  #define smp_read_barrier_depends()	read_barrier_depends()
>  #else
>  #define smp_mb()	barrier()