Subject: Re: hackbench regression due to commit 9dfc6e68bfe6e

From: David Miller <[email-masked]>
Date: Thu, 08 Apr 2010 00:20:11 -0700 (PDT)

From: David Miller <[email-masked]>
Date: Thu, 08 Apr 2010 00:05:57 -0700 (PDT)

> From: Eric Dumazet <[email-masked]>
> Date: Thu, 08 Apr 2010 09:00:19 +0200
> 
>> If run individually, the tests results are more what we would expect
>> (slow), but if machine runs the two set of process concurrently, each
>> group runs much faster...
> 
> BTW, I just discovered (thanks to the function graph tracer, woo hoo!)
> that loopback TCP packets get fully checksum validated on receive.
> 
> I'm trying to figure out why skb->ip_summed ends up being
> CHECKSUM_NONE in tcp_v4_rcv() even though it gets set to
> CHECKSUM_PARTIAL in tcp_sendmsg().

Ok, it looks like it's only ACK packets that have this problem,
but still :-)

It's weird that we have a special ip_dev_loopback_xmit() for for
ip_mc_output() NF_HOOK()s, which forces skb->ip_summed to
CHECKSUM_UNNECESSARY, but the actual normal loopback xmit doesn't
do that...