netdev - Re: TSO/GRO/LRO/somethingO breaks LVS on 2.6.36

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Date:	Fri, 3 Dec 2010 21:36:19 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Simon Kirby <sim@...tway.ca>, netdev@...r.kernel.org
Subject: Re: TSO/GRO/LRO/somethingO breaks LVS on 2.6.36

On Fri, Dec 03, 2010 at 12:29:03PM +0100, Eric Dumazet wrote:
> Le vendredi 03 décembre 2010 à 02:34 -0800, Simon Kirby a écrit :
> > Hello!
> > 
> > We upgraded some LVS (DR) servers from 2.6.35 to 2.6.36 on tg3 cards
> > (partno(BCM95721) rev 4201) with VLAN tags in use, to think that
> > everything looked great, but in fact...
> > 
> > LVS was receiving magically-merged TCP packets which it tried to forward
> > on to the real server, only to get annoyed at itself for trying to
> > forward a packet bigger than the device MTU:
> > 
> > IP A.47376 > B.529: . 175488:176936(1448) ack 1 win 92 <nop,nop,timestamp 52737308 29656081>
> > IP A.47376 > B.529: . 176936:179832(2896) ack 1 win 92 <nop,nop,timestamp 52737325 29656098>
> > IP B > A: ICMP B unreachable - need to frag (mtu 1500), length 556
> > 
> 
> Hi Simon
> 
> This is a tcpdump on A ?
> Could you take it also on B ?
> 
> tcpdump displays large buffers, but they should be split (of course)
> when sent on wire.
> 
> > This caused packet loss for any merged frames, which caused abysmal
> > performance for uploads via the LVS server.  Local performance to or
> > from the box is still fine, because the stack doesn't care, only the
> > forwarding part of LVS is running into the problem.
> > 
> > Furthermore, disabling _everything_ reported by ethtool -k doesn't seem
> > to change the result, even if I down/up the interface after, and even if
> > I try on every single interface including the VLANned ones.  This seems
> > to be another bug.  Reverting to 2.6.35 makes it all work again.
> > 
> > Possibly related to commit 7fe876af921d1d2bc8353e0062c10ff35e902653
> > 
> > So how should this be fixed?  Should LVS be taught to fragment, or must
> > we disable the merging in this case?  It seems like it would work well if
> > the sending side could do the same offload in reverse, but I'm not sure
> > if that would be possible.
> > 
> > Simon-
> 
> 
> I believe Simon Horman has some patches for GRO and LVS.
> 
> Please send the results of "ethtool -k eth0" on all your nics / vlans ?
> 
> For TSO, I am not sure why and where it could matter...

There is a patch to teach LVS how to cope with GRO in nf-next-2.6
and I expect it to be included in 2.6.38. The patch is "ipvs: allow
transmit of GRO aggregated skbs" and perhaps it should be considered
for 2.6.37 and stable. In general the work around is to disable GRO.

The patch does not resolve the incompatibility of LVS with LRO.
The work around there is to disable LRO. I'm not entirely sure
how to teach LVS to disable LRO automatically, or if its desirable.

Simon, you mention that you disabled everything with ethtool, but the
tcpdump above shows a 2896 byte packet, which seems that GRO (or LRO?) is
active. So perhaps as you speculate that is a bug

I will prepare a backport of the "ipvs: allow transmit of GRO aggregated
skbs" patch to v2.6.36 and post it shortly.  Testing to see if that
resolves the problem that you are seeing would probably be a good start.

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