Re: [RFC bpf-next 0/4] Add XDP rx hw hints support performing XDP_REDIRECT

Jesper Dangaard Brouer
2024-10-04T10:38:34Z

On 04/10/2024 04.13, Daniel Xu wrote:
> On Thu, Oct 03, 2024 at 01:26:08PM GMT, Stanislav Fomichev wrote:
>> On 10/03, Arthur Fabre wrote:
>>> On Thu Oct 3, 2024 at 12:49 AM CEST, Stanislav Fomichev wrote:
>>>> On 10/02, Toke Høiland-Jørgensen wrote:
>>>>> Stanislav Fomichev <stfomichev@gmail.com> writes:
>>>>>
>>>>>> On 10/01, Toke Høiland-Jørgensen wrote:
>>>>>>> Lorenzo Bianconi <lorenzo@kernel.org> writes:
>>>>>>>
>>>>>>>>> On Mon Sep 30, 2024 at 1:49 PM CEST, Lorenzo Bianconi wrote:
>>>>>>>>>>> Lorenzo Bianconi <lorenzo@kernel.org> writes:
>>>>>>>>>>>
[...]
>>>>>>>>>>>>
>>>>>>>>>>>> I like this 'fast' KV approach but I guess we should really evaluate its
>>>>>>>>>>>> impact on performances (especially for xdp) since, based on the kfunc calls
>>>>>>>>>>>> order in the ebpf program, we can have one or multiple memmove/memcpy for
>>>>>>>>>>>> each packet, right?
>>>>>>>>>>>
>>>>>>>>>>> Yes, with Arthur's scheme, performance will be ordering dependent. Using

I really like the *compact* Key-Value (KV) store idea from Arthur.
- The question is it is fast enough?

I've promised Arthur to XDP micro-benchmark this, if he codes this up to
be usable in the XDP code path. Listening to the LPC recording I heard
that Alexei also saw potential and other use-case for this kind of
fast-and-compact KV approach.

I have high hopes for the performance, as Arthur uses POPCNT instruction
which is *very* fast[1]. I checked[2] AMD Zen 3 and 4 have Ops/Latency=1
and Reciprocal throughput 0.25.

[1] https://www.agner.org/optimize/blog/read.php?i=853#848
[2] https://www.agner.org/optimize/instruction_tables.pdf

[...]
>>> There are two different use-cases for the metadata:
>>>
>>> * "Hardware" metadata (like the hash, rx_timestamp...). There are only a
>>> few well known fields, and only XDP can access them to set them as
>>> metadata, so storing them in a struct somewhere could make sense.
>>>
>>> * Arbitrary metadata used by services. Eg a TC filter could set a field
>>> describing which service a packet is for, and that could be reused for
>>> iptables, routing, socket dispatch...
>>> Similarly we could set a "packet_id" field that uniquely identifies a
>>> packet so we can trace it throughout the network stack (through
>>> clones, encap, decap, userspace services...).
>>> The skb->mark, but with more room, and better support for sharing it.
>>>
>>> We can only know the layout ahead of time for the first one. And they're
>>> similar enough in their requirements (need to be stored somewhere in the
>>> SKB, have a way of retrieving each one individually, that it seems to
>>> make sense to use a common API).
>>
>> Why not have the following layout then?
>>
>> +---------------+-------------------+----------------------------------------+------+
>> | more headroom | user-defined meta | hw-meta (potentially fixed skb format) | data |
>> +---------------+-------------------+----------------------------------------+------+
>> ^ ^
>> data_meta data
>>
>> You obviously still have a problem of communicating the layout if you
>> have some redirects in between, but you, in theory still have this
>> problem with user-defined metadata anyway (unless I'm missing
>> something).
>>

Hmm, I think you are missing something... As far as I'm concerned we are
discussing placing the KV data after the xdp_frame, and not in the XDP
data_meta area (as your drawing suggests). The xdp_frame is stored at
the very top of the headroom. Lorenzo's patchset is extending struct
xdp_frame and now we are discussing to we can make a more flexible API
for extending this. I understand that Toke confirmed this here [3]. Let
me know if I missed something :-)

[3] https://lore.kernel.org/all/874j62u1lb.fsf@toke.dk/

As part of designing this flexible API, we/Toke are trying hard not to
tie this to a specific data area. This is a good API design, keeping it
flexible enough that we can move things around should the need arise.

I don't think it is viable to store this KV data in XDP data_meta area,
because existing BPF-prog's already have direct memory (write) access
and can change size of area, which creates too much headache with
(existing) BPF-progs creating unintentional breakage for the KV store,
which would then need extensive checks to handle random corruptions
(slowing down KV-store code).

--Jesper