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Old 11-21-2008, 10:44 AM
martin f krafft
 
Default Bug#506419: forcedeth at fault, checksum offloading

found 506419 2.6.26-8
thanks

Bastian Blank helpfully diagnosed the problem to be with the
forcedeth driver. He pointed me at commit edcfe5f[0], which should
be in 2.6.26.4, but apparently it's either not present in Debian's
2.6.26-10 package, or it does not fix the bug entirely.

0. http://repo.or.cz/w/linux-2.6.git?a=commitdiff;h=edcfe5f7e307846e578fb88d69f a27051fded0ab

The issue doesn't seem new[1] and Bastian thinks it might have to
do with bridging, which is in fact being used on my system.

1. http://kerneltrap.org/mailarchive/linux-netdev/2008/10/16/3674714

khyber:~# brctl show
bridge name bridge id STP enabled interfaces
br0 8000.003048c2f5e8 no eth0

He suggests it's to do with checksum offloading and asked me to
experiment with disabling the offloading:

khyber:~# ethtool --show-offload eth0
Offload parameters for eth0:
rx-checksumming: on
tx-checksumming: on
scatter-gather: on
tcp segmentation offload: on
udp fragmentation offload: off
generic segmentation offload: on

khyber:~# ethtool --show-offload br0
Offload parameters for br0:
Cannot get device rx csum settings: Operation not supported
rx-checksumming: off
tx-checksumming: on
scatter-gather: on
tcp segmentation offload: on
udp fragmentation offload: off
generic segmentation offload: off

The problem goes away if I turn all features off, on either one
interface, or on both together, e.g.

khyber:~# i=eth0; set -x; for s in rx tx sgo tso ufo gso; do ethtool -K $i $s off; done; set +x
+ for s in rx tx sgo tso ufo gso
+ ethtool -K eth0 rx off
+ for s in rx tx sgo tso ufo gso
+ ethtool -K eth0 tx off
+ for s in rx tx sgo tso ufo gso
+ ethtool -K eth0 sgo off
no offload settings changed
+ for s in rx tx sgo tso ufo gso
+ ethtool -K eth0 tso off
+ for s in rx tx sgo tso ufo gso
+ ethtool -K eth0 ufo off
Cannot set device udp large send offload settings: Operation not supported
+ for s in rx tx sgo tso ufo gso
+ ethtool -K eth0 gso off
+ set +x

The problem can indeed be fixed with

ethtool -K eth0 tx off

or so it seems. Unfortunately, this reduces the throughput rate by
a noticeable amount.

The problem also exists with 2.6.26-8, although I have not noticed
it before last night's upgrade to -10.

Thanks, Bastian,

--
.'`. martin f. krafft <madduck@debian.org>
: :' : proud Debian developer, author, administrator, and user
`. `'` http://people.debian.org/~madduck - http://debiansystem.info
`- Debian - when you have better things to do than fixing systems
 

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