[PATCH RFC v3 0/5] Support fraglist GRO/GSO

[Steffen Klassert <steffen.klassert@secunet.com>]

This patchset adds support to do GRO/GSO by chaining packets
of the same flow at the SKB frag_list pointer. This avoids
the overhead to merge payloads into one big packet, and
on the other end, if GSO is needed it avoids the overhead
of splitting the big packet back to the native form.

Patch 1 Enables UDP GRO by default.

Patch 2 adds a netdev feature flag to enable listifyed GRO,
this implements one of the configuration options discussed
at netconf 2019.

Patch 3 adds a netdev software feature set that defaults to off
and assigns the new listifyed GRO feature flag to it.

Patch 4 adds the core infrastructure to do fraglist GRO/GSO.

Patch 5 enables UDP to use fraglist GRO/GSO if configured and no
GRO supported socket is found.

I have only meaningful forwarding performance measurements.
I did some tests for the local receive path with netperf and iperf,
but in this case the sender that generates the packets is the
bottleneck. So the benchmarks are not that meaningful for the
receive path.

Paolo Abeni did some benchmarks of the local receive path for the v2
version of this pachset, results can be found here:

https://www.spinics.net/lists/netdev/msg551158.html

I used my IPsec forwarding test setup for the performance measurements:

           ------------         ------------
        -->| router 1 |-------->| router 2 |--
        |  ------------         ------------  |
        |                                     |
        |       --------------------          |
        --------|Spirent Testcenter|<----------
                --------------------

net-next (September 7th):

Single stream UDP frame size 1460 Bytes: 1.161.000 fps (13.5 Gbps).

----------------------------------------------------------------------

net-next (September 7th) + standard UDP GRO/GSO:

Single stream UDP frame size 1460 Bytes: 1.801.000 fps (21 Gbps).

----------------------------------------------------------------------

net-next (September 7th) + fraglist UDP GRO/GSO:

Single stream UDP frame size 1460 Bytes: 2.860.000 fps (33.4 Gbps).

-----------------------------------------------------------------------

Changes from v1:

- Add IPv6 support.
- Split patchset to enable UDP GRO by default before adding
  fraglist GRO support.
- Mark fraglist GRO packets as CHECKSUM_NONE.
- Take a refcount on the first segment skb when doing fraglist
  segmentation. With this we can use the same error handling
  path as with standard segmentation.

Changes from v2:

- Add a netdev feature flag to configure listifyed GRO.
- Fix UDP GRO enabling for IPv6.
- Fix a rcu_read_lock() imbalance.
- Fix error path in skb_segment_list().