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

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Date:	Wed, 4 Dec 2013 01:09:06 +0200
From:	Or Gerlitz <or.gerlitz@...il.com>
To:	Joseph Gasparakis <joseph.gasparakis@...el.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Jerry Chu <hkchu@...gle.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Pravin B Shelar <pshelar@...ira.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>
Subject: Re: vxlan/veth performance issues on net.git + latest kernels

On Wed, Dec 4, 2013 at 1:13 AM, Joseph Gasparakis
<joseph.gasparakis@...el.com> wrote:
>
>
> On Tue, 3 Dec 2013, Or Gerlitz wrote:
>
>> On Tue, Dec 3, 2013 at 11:11 PM, Joseph Gasparakis
>> <joseph.gasparakis@...el.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

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