rte_flow API change request: tunnel info restoration is too slow

[Ilya Maximets <i.maximets@ovn.org>]

Hi, everyone.

After implementing support for tunnel offloading in OVS we faced a
significant performance issue caused by the requirement to call
rte_flow_get_restore_info() function on a per-packet basis.

The main problem is that once the tunnel offloading is configured,
there is no way for the application to tell if a particular packet
was partially processed in the hardware and the tunnel info has to
be restored.  What we have to do right now is to call the
rte_flow_get_restore_info() unconditionally for every packet.  The
result of that call says if we have the tunnel info or not.

rte_flow_get_restore_info() call itself is very heavy.  It is at
least a couple of indirect function calls and the device lock
on the application side (not-really-thread-safety of the rte_flow
API is a separate topic).  Internal info lookup inside the driver
also costs a lot (depends on a driver).

It has been measured that having this call on a per-packet basis can
reduce application performance (OVS) by a factor of 3 in some cases:
  https://mail.openvswitch.org/pipermail/ovs-dev/2021-November/389265.html
  https://github.com/openvswitch/ovs/commit/6e50c1651869de0335eb4b7fd0960059c5505f5c
(Above patch avoid the problem in a hacky way for devices that doesn't
 support tunnel offloading, but it's not applicable to situation
 where device actually supports it, since the API has to be called.)

Another tricky part is that we have to call rte_flow_get_restore_info()
before checking other parts of the mbuf, because mlx5 driver, for
example, re-uses the mbuf.hash.fdir value for both tunnel info
restoration and classification offloading, so the application has
no way to tell which one is used right now and has to call the
restoration API first in order to find out.


What we need:

A generic and fast (couple of CPU cycles) API that will clearly say
if the heavy rte_flow_get_restore_info() has to be called for a
particular packet or not.  Ideally, that should be a static mbuf
flag that can be easily checked by the application.

Calls inside the device driver are way too slow for that purpose,
especially if they are not fully thread-safe, or require complex
lookups or calculations.

I'm assuming here that packets that really need the tunnel info
restoration should be fairly rare.


Current state:

Currently, the get_restore_info() API is implemented only for mlx5
and sfc drivers, AFAICT.  SFC driver is already using mbuf flag, but
it's dynamic and not exposed to the application.  MLX5 driver re-uses
mbuf.hash.fdir value and performs a heavy lookup inside the driver.

For now OVS doesn't support tunnel offload with DPDK formally, the
code in OVS is under the experimental API ifdef and not compiled-in
by default.

//Let me know if there is more formal way to submit such requests.

Best regards, Ilya Maximets.