netdev - [PATCH] eth: Declare an optimized compare_ether_addr

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Date:	Sat, 22 Nov 2008 08:19:17 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	"David S. Miller" <davem@...emloft.net>
CC:	Linux Netdev List <netdev@...r.kernel.org>
Subject: [PATCH] eth: Declare an optimized compare_ether_addr_64bits() function

Hello David, this is a resend of a patch previously sent in a 
"tbench regression ..." thread on lkml

We should also address the problem of skb_pull(skb, ETH_HLEN);
in eth_type_trans() :

Being not inlined, this force eth_type_trans() to be a non
leaf function, that cost precious cpu cycles on many arches.

Thank you

[PATCH] eth: Declare an optimized compare_ether_addr_64bits() function

Linus mentioned we could try to perform long word operations, even
on potentially unaligned addresses, on x86 at least.

I tried this idea and got nice assembly on 32 bits:

158:   33 82 38 01 00 00       xor    0x138(%edx),%eax
15e:   33 8a 34 01 00 00       xor    0x134(%edx),%ecx
164:   c1 e0 10                shl    $0x10,%eax
167:   09 c1                   or     %eax,%ecx
169:   74 0b                   je     176 <eth_type_trans+0x87>

And very nice assembly on 64 bits of course (one xor, one shl)

Nice oprofile improvement in eth_type_trans(), 0.17 % instead of 0.41 %,
expected since we remove 8 instructions on a fast path.

This patch implements a compare_ether_addr_64bits() function,
that handles the case of x86 cpus, but might be used on other arches as well,
if their potential misaligned long word reads are not expensive.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
---
 include/linux/etherdevice.h |   41 ++++++++++++++++++++++++++++++++++
 net/ethernet/eth.c          |    4 +--
 2 files changed, 43 insertions(+), 2 deletions(-)

View attachment "compare_ether_addr_64bits.patch" of type "text/plain" (2439 bytes)