Re: vxlan/veth performance issues on net.git + latest kernels

[Joseph Gasparakis <joseph.gasparakis@intel.com>]

On Tue, 3 Dec 2013, Or Gerlitz wrote:

> On Wed, Dec 4, 2013 at 1:13 AM, Joseph Gasparakis
> <joseph.gasparakis@intel.com> wrote:
> >
> >
> > On Tue, 3 Dec 2013, Or Gerlitz wrote:
> >
> >> On Tue, Dec 3, 2013 at 11:11 PM, Joseph Gasparakis
> >> <joseph.gasparakis@intel.com> wrote:
> >>
> >> >>> lack of GRO : receiver seems to not be able to receive as fast as you want.
> >> >>>>      TCPOFOQueue: 3167879
> >> >>> So many packets are received out of order (because of losses)
> >>
> >> >> I see that there's no GRO also for the non-veth tests which involve
> >> >> vxlan, and over there the receiving side is capable to consume the
> >> >> packets, do you have rough explaination why adding veth to the chain
> >> >> is such game changer which makes things to start falling out?
> >>
> >> > I have seen this before. Here are my findings:
> >> >
> >> > The gso_type is different if the skb comes from veth or not. From veth,
> >> > you will see the SKB_GSO_DODGY set. This breaks things as when the
> >> > skb with DODGY set moves from vxlan to the driver through dev_xmit_hard,
> >> > the stack drops it silently. I never got the time to find the root cause
> >> > for this, but I know it causes re-transmissions and big performance
> >> > degregation.
> >> >
> >> > I went as far as just quickly hacking a one liner unsetting the DODGY bit
> >> > in vxlan.c and that bypassed the issue and recovered the performance
> >> > problem, but obviously this is not a real fix.
> >>
> >> thanks for the heads up, few quick questions/clafications --
> >>
> >> -- you are talking on drops done @ the sender side, correct? Eric was
> >> saying we have evidences that the drops happen on the receiver.
> >
> > I am *guessing* drops on the Rx are due to the drops at the Tx. See my
> > answer to your next question for more info.
> >
> >>
> >> -- without the hack you did, still packets are sent/received, so what
> >> makes the stack to drop only some of them?
> >>
> >
> > What I had seen is GSOs getting dropped on the Tx side. Basically the GSOs
> > never made it to the driver, they were broken into non GSO smaller skbs by
> > the stack. I think the stack is not handling well the GSO with the DODGY
> > bit set, and that causes it to maybe partially the packet to be emitted,
> > causing the re-transmits (and maybe the drops on your Rx end)? Of course
> > all this is speculation, the fact that I know is that as soon as I was
> > forcing the gso type I saw offloaded VXLAN encapsulated traffic at decent speeds.
> >
> >> -- why packets coming from veth would have the SKB_GSO_DODGY bit set?
> >
> > That is something I would love to know too. I am guessing this is a way
> > for the VM to say it is a non-trusted packet? And maybe all this can be
> > fixed by maybe setting something on the VM through a userspace tool that
> > will stop the veth to set the DODGY bit?
> >
> >>
> >> -- so where is now (say net.git or 3.12.x) this one line you commented
> >> out? I don't see in vxlan.c or in ip_tunnel_core.c / ip_tunnel.c
> >> explicit setting of SKB_GSO_DODGY
> >
> > I did not commit it, as this was just a workaround to prove to myself that
> > the problem I was seing was due to the gso_type, and it would actually
> > just hide the problem and not give a proper solution to it.
> >
> >>
> >> Also, I am pretty sure the problem exists also when sending/receiving
> >> guest traffic through tap/macvtap <--> vhost/virtio-net and friends, I
> >> just sticked to the veth flavour b/c its one (== the hypervisor)
> >> network stack to debug and not two (+ the guest one).
> 
> understood, can you point the line/area you hacked, I'd like to try it
> too and see the impact

I was printing the gso_type in vxlan_xmit_skb(), right before 
iptunnel_xmit() gets called (I was focus UDPv4 encap only). Then I saw the 
gso_type was different when a VM was involved and when it was not 
(although I was transmitting exactly the same packet), and then I replaced 
my printk with something like skb_shinfo(skb)->gso_type = <the gso type I had
for non-VM skb> and it all worked.

Then I looked into what was different between the two gso_types and the 
only difference was that SKB_GSO_DODGY was set when Tx'ing from the VM.
I am sure I could have been more delicate with the aproach, but hey, it
worked for me.

I would be curious to see if this is the same issue as mine. It seems like 
it is.

> 
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