linux-kernel - Re: change last level cache alignment on x86?

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Date:	Fri, 02 Mar 2012 15:30:25 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	tglx@...utronix.de
Cc:	hpa@...or.com, mingo@...hat.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	x86@...nel.org, asit.k.mallick@...el.com
Subject: Re: change last level cache alignment on x86?

On Thu, 2012-03-01 at 16:33 +0800, Alex,Shi wrote:
> Currently last level defined in kernel is still 128 bytes, but actually
> I checked intel's core2, NHM, SNB, atom, serial platforms, all of them
> are using 64 bytes. 
> I did not get detailed info on AMD platforms. Guess someone like to give
> the info here. So, Is if it possible to do the similar following changes
> to use 64 byte cache alignment in kernel?
> 
> ===
> diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
> index 3c57033..f342a5a 100644
> --- a/arch/x86/Kconfig.cpu
> +++ b/arch/x86/Kconfig.cpu
> @@ -303,7 +303,7 @@ config X86_GENERIC
>  config X86_INTERNODE_CACHE_SHIFT
>  	int
>  	default "12" if X86_VSMP
> -	default "7" if NUMA
> +	default "7" if NUMA && (MPENTIUM4)
>  	default X86_L1_CACHE_SHIFT
>  
>  config X86_CMPXCHG

In arch/x86/include/asm/cache.h, the INTERNODE_CACHE_SHIFT macro will
transfer to '__cacheline_aligned_in_smp' finally. 

#ifdef CONFIG_X86_VSMP
#ifdef CONFIG_SMP
#define __cacheline_aligned_in_smp                                      \
        __attribute__((__aligned__(INTERNODE_CACHE_BYTES)))             \
        __page_aligned_data
#endif
#endif

look at the following contents in Kconfig.cpu, I wondering if it is
possible to remove 'default "7" if NUMA' line. Then a thin and fit cache
alignment will be potential helpful on performance.
Anyone like to give some comments? 

===
diff --git a/arch/x86/Kconfig.cpu b/arch/x86/Kconfig.cpu
index 3c57033..6443c6f 100644
--- a/arch/x86/Kconfig.cpu
+++ b/arch/x86/Kconfig.cpu
@@ -303,7 +303,6 @@ config X86_GENERIC
 config X86_INTERNODE_CACHE_SHIFT
        int
        default "12" if X86_VSMP
-       default "7" if NUMA
        default X86_L1_CACHE_SHIFT
 
 config X86_CMPXCHG
====

some contents in Kconfig.cpu: 

config X86_INTERNODE_CACHE_SHIFT
        int
        default "12" if X86_VSMP
        default "7" if NUMA && (MPENTIUM4 || MPSC)
        default X86_L1_CACHE_SHIFT

config X86_CMPXCHG
        def_bool X86_64 || (X86_32 && !M386)

config X86_L1_CACHE_SHIFT
        int
        default "7" if MPENTIUM4 || MPSC
        default "6" if MK7 || MK8 || MPENTIUMM || MCORE2 || MATOM || MVIAC7 || X86_GENERIC || GENERIC_CPU
        default "4" if MELAN || M486 || M386 || MGEODEGX1
        default "5" if MWINCHIP3D || MWINCHIPC6 || MCRUSOE || MEFFICEON || MCYRIXIII || MK6 || MPENTIUMIII || MPENTIUMII || M686 || M586MMX || M586TSC || M586 || MVIAC3_2 || MGEODE_LX


> 


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