[ovs-dev] [PATCH] [rfc] datapath: Treat GSO skbs as if they were fragments

[Jesse Gross]

On Sat, Jan 1, 2011 at 7:29 AM, Simon Horman <horms at verge.net.au> wrote:
> In dp_output_control() GSO skbs are split into fragments which are passed
> to userspace. So the resulting flow set-up by the controller (I am using
> ovs-vswitchd) is created based on a fragment. In the case of UDP and TCP,
> this means the UDP/TCP source and destination port is zero.
>
> In order for the datapath to match the resulting flow flow_extract() needs
> to treat GSO skbs as if they are fragments.  That is, set the UDP/TCP
> source and destination port to 0.
>
> A flow established for a GSO skb with this change won't match any
> subsequent non-GSO skbs, they will need to be passed to the controller and
> a new flow established. But without this change no GSO skbs will ever match
> any flow.
>
> I noticed this while using KVM using virtio with VhostNet and netperf's
> UDP_STREAM test. The result was that the test sent ~5Gbit/s but only a
> small fraction of that was received by the other side. Much less than the
> 1Gbit/s available on the physical link between the host (and guest) and the
> machine running netserver. 100% of one of the host's CPUs was consumed, 50%
> for the host and 50% for the guest.  The host consumption was contributed
> to largely by ovs-vswitchd.
>
> With this change I get a much nicer result of a fraction under 1Gbit/s sent
> and almost all packets ending up at the other end.

Hmm, you're right - this isn't currently being handled correctly.  GSO
is just a performance optimization, so we just behave the same as if
it didn't exist.

I think this change should only apply to UDP though - GSO for TCP
implies TCP segmentation, whereas as UDP is IP fragmentation.  If we
are doing segmentation we will have the TCP header, including the
ports.

Also, for consistency we should probably also set the is_frag field.