From: David S. Miller <davem@davemloft.net>
Date: Sat, 5 Mar 2011 05:24:47 +0000 (-0800)
Subject: ipv4: Optimize flow initialization in output route lookup.
X-Git-Tag: v2.6.39-rc1~468^2~156
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=44713b67db10c774f14280c129b0d5fd13c70cf2;p=pandora-kernel.git

ipv4: Optimize flow initialization in output route lookup.

We burn a lot of useless cycles, cpu store buffer traffic, and
memory operations memset()'ing the on-stack flow used to perform
output route lookups in __ip_route_output_key().

Only the first half of the flow object members even matter for
output route lookups in this context, specifically:

FIB rules matching cares about:

	dst, src, tos, iif, oif, mark

FIB trie lookup cares about:

	dst

FIB semantic match cares about:

	tos, scope, oif

Therefore only initialize these specific members and elide the
memset entirely.

On Niagara2 this kills about ~300 cycles from the output route
lookup path.

Likely, we can take things further, since all callers of output
route lookups essentially throw away the on-stack flow they use.
So they don't care if we use it as a scratch-pad to compute the
final flow key.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
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