Re: [PATCH net-next v8 00/15] Introducing P4TC

Jiri Pirko
2023-11-20T18:10:17Z

Mon, Nov 20, 2023 at 03:23:59PM CET, jhs@mojatatu.com wrote:
>On Mon, Nov 20, 2023 at 4:39 AM Jiri Pirko <jiri@resnulli.us> wrote:
>>
>> Fri, Nov 17, 2023 at 09:46:11PM CET, jhs@mojatatu.com wrote:
>> >On Fri, Nov 17, 2023 at 1:37 PM John Fastabend <john.fastabend@gmail.com> wrote:
>> >>
>> >> Jamal Hadi Salim wrote:
>> >> > On Fri, Nov 17, 2023 at 1:27 AM John Fastabend <john.fastabend@gmail.com> wrote:
>> >> > >
>> >> > > Jamal Hadi Salim wrote:
>>
>> [...]
>>
>>
>> >>
>> >> I think I'm judging the technical work here. Bullet points.
>> >>
>> >> 1. p4c-tc implementation looks like it should be slower than a
>> >> in terms of pkts/sec than a bpf implementation. Meaning
>> >> I suspect pipeline and objects laid out like this will lose
>> >> to a BPF program with an parser and single lookup. The p4c-ebpf
>> >> compiler should look to create optimized EBPF code not some
>> >> emulated switch topology.
>> >>
>> >
>> >The parser is ebpf based. The other objects which require control
>> >plane interaction are not - those interact via netlink.
>> >We published perf data a while back - presented at the P4 workshop
>> >back in April (was in the cover letter)
>> >https://github.com/p4tc-dev/docs/blob/main/p4-conference-2023/2023P4WorkshopP4TC.pdf
>> >But do note: the correct abstraction is the first priority.
>> >Optimization is something we can teach the compiler over time. But
>> >even with the minimalist code generation you can see that our approach
>> >always beats ebpf in LPM and ternary. The other ones I am pretty sure
>>
>> Any idea why? Perhaps the existing eBPF maps are not that suitable for
>> this kinds of lookups? I mean in theory, eBPF should be always faster.
>
>We didnt look closely; however, that is not the point - the point is
>the perf difference if there is one, is not big with the big win being
>proper P4 abstraction. For LPM for sure our algorithmic approach is
>better. For ternary the compute intensity in looping is better done in
>C. And for exact i believe that ebpf uses better hashing.
>Again, that is not the point we were trying to validate in those experiments..
>
>On your point of "maps are not that suitable" P4 tables tend to have
>very specific attributes (examples associated meters, counters,
>default hit and miss actions, etc).
>
>> >we can optimize over time.
>> >Your view of "single lookup" is true for simple programs but if you
>> >have 10 tables trying to model a 5G function then it doesnt make sense
>> >(and i think the data we published was clear that you gain no
>> >advantage using ebpf - as a matter of fact there was no perf
>> >difference between XDP and tc in such cases).
>> >
>> >> 2. p4c-tc control plan looks slower than a directly mmaped bpf
>> >> map. Doing a simple update vs a netlink msg. The argument
>> >> that BPF can't do CRUD (which we had offlist) seems incorrect
>> >> to me. Correct me if I'm wrong with details about why.
>> >>
>> >
>> >So let me see....
>> >you want me to replace netlink and all its features and rewrite it
>> >using the ebpf system calls? Congestion control, event handling,
>> >arbitrary message crafting, etc and the years of work that went into
>> >netlink? NO to the HELL.
>>
>> Wait, I don't think John suggests anything like that. He just suggests
>> to have the tables as eBPF maps.
>
>What's the difference? Unless maps can do netlink.
>
>> Honestly, I don't understand the
>> fixation on netlink. Its socket messaging, memcpies, processing
>> overhead, etc can't keep up with mmaped memory access at scale. Measure
>> that and I bet you'll get drastically different results.
>>
>> I mean, netlink is good for a lot of things, but does not mean it is an
>> universal answer to userspace<->kernel data passing.
>
>Here's a small sample of our requirements that are satisfied by
>netlink for P4 object hierarchy[1]:
>1. Msg construction/parsing
>2. Multi-user request/response messaging

What is actually a usecase for having multiple users program p4 pipeline
in parallel?

>3. Multi-user event subscribe/publish messaging

Same here. What is the usecase for multiple users receiving p4 events?

>
>I dont think i need to provide an explanation on the differences here
>visavis what ebpf system calls provide vs what netlink provides and
>how netlink is a clear fit. If it is not clear i can give more

It is not :/

>breakdown. And of course there's more but above is a good sample.
>
>The part that is taken for granted is the control plane code and
>interaction which is an extremely important detail. P4 Abstraction
>requires hierarchies with different compiler generated encoded path
>ids etc. This ID mapping gets exacerbated by having multitudes of P4

Why the actual eBFP mapping does not serve the same purpose as ID?
ID:mapping
1 :1
?

>programs which have different requirements. Netlink is a natural fit
>for this P4 abstraction. Not to mention the netlink/tc path (and in
>particular the ID mapping) provides a conduit for offload when that is
>needed.
>eBPF is just a tool - and the objects are intended to be generic - and
>i dont see how any of this could be achieved without retooling to make
>it more specific to P4.
>
>cheers,
>jamal
>
>
>
>>
>> >I should note: that there was an interesting talk at netdevconf 0x17
>> >where the speaker showed the challenges of dealing with ebpf on "day
>> >two" - slides or videos are not up yet, but link is:
>> >https://netdevconf.info/0x17/sessions/talk/is-scaling-ebpf-easy-yet-a-small-step-to-one-server-but-giant-leap-to-distributed-network.html
>> >The point the speaker was making is it's always easy to whip an ebpf
>> >program that can slice and dice packets and maybe even flush LEDs but
>> >the real work and challenge is in the control plane. I agree with the
>> >speaker based on my experiences. This discussion of replacing netlink
>> >with ebpf system calls is absolutely a non-starter. Let's just end the
>> >discussion and agree to disagree if you are going to keep insisting on
>> >that.
>>
>>
>> …