netdev - Re: [PATCH v3] rps: optimize rps_get

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Message-ID: <k2h412e6f7f1004250103g3fbce599r8337fdb781317158@mail.gmail.com>
Date:	Sun, 25 Apr 2010 16:03:03 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	therbert@...gle.com, eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH v3] rps: optimize rps_get_cpu()

On Sun, Apr 25, 2010 at 3:48 PM, David Miller <davem@...emloft.net> wrote:
>
> Nevermind, currently we only depend upon the addresses in struct sock
> being 64-bit aligned not the protocol headers.
>
> It shouldn't be hard to make the protocol header addresses 64-bit
> aligned too.  Simply setting the default NET_IP_ALIGN to '6' instead
> of '2' ought to be sufficient.
>
> skb->data upon alloc_skb() is 64-bit aligned.
>
> So if we skb_reserve(NET_IP_ALIGN '6'), then we have the ethernet
> header (14 bytes).  And since 'saddr' is 12 bytes into struct iphdr it
> will be (6 + 14 + 12) == 32 bytes in from the original 64-bit aligned
> skb->data.
>
> Therefore, since skb->data is 64-bit aligned, skb->data plus a
> multiple of 8 (which 32 is) will also be 64-bit aligned, and that
> means iph->saddr will be 64-bit aligned.
>

But if there is a vlan header, extra 4-bytes are appended to the
ethernet header, so the addresses aren't aligned to 64-bit boundary
when we set NET_IP_ALIGN to 6.


-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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