netdev - small RPS cache for fragments?

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Message-Id: <20110517.143342.1566027350038182221.davem@davemloft.net>
Date:	Tue, 17 May 2011 14:33:42 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	netdev@...r.kernel.org
Subject: small RPS cache for fragments?

It seems to me that we can solve the UDP fragmentation problem for
flow steering very simply by creating a (saddr/daddr/IPID) entry in a
table that maps to the corresponding RPS flow entry.

When we see the initial frag with the UDP header, we create the
saddr/daddr/IPID mapping, and we tear it down when we hit the
saddr/daddr/IPID mapping and the packet has the IP_MF bit clear.

We only inspect the saddr/daddr/IPID cache when iph->frag_off is
non-zero.

It's best effort and should work quite well.

Even a one-behind cache, per-NAPI instance, would do a lot better than
what happens at the moment.  Especially since the IP fragments mostly
arrive as one packet train.
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