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

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Message-Id: <20100425.004842.225645379.davem@davemloft.net>
Date:	Sun, 25 Apr 2010 00:48:42 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	xiaosuo@...il.com
Cc:	therbert@...gle.com, eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH v3] rps: optimize rps_get_cpu()

From: David Miller <davem@...emloft.net>
Date: Sun, 25 Apr 2010 00:38:34 -0700 (PDT)

> From: Changli Gao <xiaosuo@...il.com>
> Date: Sun, 25 Apr 2010 14:48:49 +0800
> 
>> Are they always aligned to 64-bit boundary? I don't think so.
> 
> If not than TCP stack should be crashing for past 15 years.

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.
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