From: Eric Dumazet <edumazet@google.com>
Date: Mon, 24 Mar 2014 02:51:36 +0000 (-0700)
Subject: net: optimize csum_replace2()
X-Git-Tag: v3.15-rc1~113^2~106
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=99f0b958b194f7d88973f1c2190d207e0a2c7e79;p=pandora-kernel.git

net: optimize csum_replace2()

When changing one 16bit value by another in IP header, we can adjust
the IP checksum by doing a simple operation described in RFC 1624, as
reminded by David.

csum_partial() is a complex function on x86_64, not really suited for
small number of checksummed bytes.

I spotted csum_partial() being in the top 20 most consuming functions
(more than 1 %) in a GRO workload, which was rather unexpected.

The caller was inet_gro_complete() doing a csum_replace2() when
building the new IP header for the GRO packet.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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