Re: rte_flow API change request: tunnel info restoration is too slow

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

On 3/16/22 10:41, Thomas Monjalon wrote:
> 15/03/2022 23:12, Ilya Maximets:
>> 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.
> 
> A dynamic mbuf flag, defined in the API, looks to be a good idea.

Makes sense.  OTOH, I'm not sure what is the profit of having it
dynamically allocated if it will need to be always defined.
But, well, it doesn't really matter, I guess.

> 
>> 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.
> 
> What is this flag?

SFC driver defines a dynamic field 'rte_net_sfc_dynfield_ft_id'
and the corresponding flag 'rte_net_sfc_dynflag_ft_id_valid' to
check if the field contains a valid data.

> 
>> MLX5 driver re-uses mbuf.hash.fdir value
>> and performs a heavy lookup inside the driver.
> 
> We should avoid re-using a field.

+1 from me, but I'm not familiar with the mlx5 driver enough
to tell how to change it.

> 
>> 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.
> 
> That's a well written request, thanks.
> If you are looking for something more formal,
> it could be a patch in the API.

I'm not looking for now. :)
I think we need an agreement from driver maintainers first.  And
I don't think we can introduce such API change without changing
drivers first, because otherwise we'll end up with the 'has_vlan'
situation and the broken offloading support.

Best regards, Ilya Maximets.