netdev - Re: [net-next-2.6 PATCH 2/2] x86: Align skb w/ start of cache line on newer core 2/Xeon Arch

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Date:	Thu, 03 Jun 2010 00:44:10 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Cc:	davem@...emloft.net, mingo@...hat.com, tglx@...utronix.de,
	hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, gospo@...hat.com,
	Alexander Duyck <alexander.h.duyck@...el.com>
Subject: Re: [net-next-2.6 PATCH 2/2] x86: Align skb w/ start of cache line
 on newer core 2/Xeon Arch

Le mercredi 02 juin 2010 à 15:25 -0700, Jeff Kirsher a écrit :
> From: Alexander Duyck <alexander.h.duyck@...el.com>
> 
> x86 architectures can handle unaligned accesses in hardware, and it has
> been shown that unaligned DMA accesses can be expensive on Nehalem
> architectures.  As such we should overwrite NET_IP_ALIGN and NET_SKB_PAD
> to resolve this issue.
> 
> Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
> ---
> 
>  arch/x86/include/asm/system.h |   12 ++++++++++++
>  1 files changed, 12 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
> index b8fe48e..8acb44e 100644
> --- a/arch/x86/include/asm/system.h
> +++ b/arch/x86/include/asm/system.h
> @@ -457,4 +457,16 @@ static inline void rdtsc_barrier(void)
>  	alternative(ASM_NOP3, "lfence", X86_FEATURE_LFENCE_RDTSC);
>  }
>  
> +#ifdef CONFIG_MCORE2
> +/*
> + * We handle most unaligned accesses in hardware.  On the other hand
> + * unaligned DMA can be quite expensive on some Nehalem processors.
> + *
> + * Based on this we disable the IP header alignment in network drivers.
> + * We also modify NET_SKB_PAD to be a cacheline in size, thus maintaining
> + * cacheline alignment of buffers.
> + */
> +#define NET_IP_ALIGN	0
> +#define NET_SKB_PAD	L1_CACHE_BYTES
> +#endif
>  #endif /* _ASM_X86_SYSTEM_H */
> 
> --

But... L1_CACHE_BYTES is 64 on MCORE2, so this matches current
NET_SKB_PAD definition...

#ifndef NET_SKB_PAD
#define NET_SKB_PAD 64
#endif



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