Subject: Re: [RFC Patch net-next] net_sched: introduce eBPF based Qdisc

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 03 Sep 2021 00:27:52 +0200

John Fastabend <[email-masked]> writes:

> Toke Høiland-Jørgensen wrote:
>> Martin KaFai Lau <[email-masked]> writes:
>> 
>> > On Wed, Sep 01, 2021 at 12:42:03PM +0200, Toke Høiland-Jørgensen wrote:
>> >> John Fastabend <[email-masked]> writes:
>> >> 
>> >> > Cong Wang wrote:
>> >> >> On Tue, Aug 24, 2021 at 4:47 PM Martin KaFai Lau <[email-masked]> wrote:
>> >> >> > Please explain more on this.  What is currently missing
>> >> >> > to make qdisc in struct_ops possible?
>> >> >> 
>> >> >> I think you misunderstand this point. The reason why I avoid it is
>> >> >> _not_ anything is missing, quite oppositely, it is because it requires
>> >> >> a lot of work to implement a Qdisc with struct_ops approach, literally
>> >> >> all those struct Qdisc_ops (not to mention struct Qdisc_class_ops).
>> >> >> WIth current approach, programmers only need to implement two
>> >> >> eBPF programs (enqueue and dequeue).
>> > _if_ it is using as a qdisc object/interface,
>> > the patch "looks" easier because it obscures some of the ops/interface
>> > from the bpf user.  The user will eventually ask for more flexibility
>> > and then an on-par interface as the kernel's qdisc.  If there are some
>> > common 'ops', the common bpf code can be shared as a library in userspace
>> > or there is also kfunc call to call into the kernel implementation.
>> > For existing kernel qdisc author,  it will be easier to use the same
>> > interface also.
>> 
>> The question is if it's useful to provide the full struct_ops for
>> qdiscs? Having it would allow a BPF program to implement that interface
>> towards userspace (things like statistics, classes etc), but the
>> question is if anyone is going to bother with that given the wealth of
>> BPF-specific introspection tools already available?
>
> If its a map value then you get all the goodness with normal map
> inspection.

Yup, exactly, so why bother with struct_ops to implement all the other
qdisc ops (apart from enqueue/dequeue)?

>> My hope is that we can (longer term) develop some higher-level tools to
>> express queueing policies that can then generate the BPF code needed to
>> implement them. Or as a start just some libraries to make this easier,
>> which I think is also what you're hinting at here? :)
>
> The P4 working group has thought about QOS and queuing from P4 side if
> you want to think in terms of a DSL. Might be interesting and have
> some benefits if you want to drop into hardware offload side. For example
> compile to XDP for fast CPU architectures, Altera/Xilinx backend for FPGA or
> switch silicon for others. This was always the dream on my side maybe
> we've finally got close to actualizing it, 10 years later ;)

Yup, would love to see this! Let's just hope it doesn't take another
decade ;)

>> >> > Another idea. Rather than work with qdisc objects which creates all
>> >> > these issues with how to work with existing interfaces, filters, etc.
>> >> > Why not create an sk_buff map? Then this can be used from the existing
>> >> > egress/ingress hooks independent of the actual qdisc being used.
>> >> 
>> >> I agree. In fact, I'm working on doing just this for XDP, and I see no
>> >> reason why the map type couldn't be reused for skbs as well. Doing it
>> >> this way has a couple of benefits:
>> >> 
>> >> - It leaves more flexibility to BPF: want a simple FIFO queue? just
>> >>   implement that with a single queue map. Or do you want to build a full
>> >>   hierarchical queueing structure? Just instantiate as many queue maps
>> >>   as you need to achieve this. Etc.
>> > Agree.  Regardless how the interface may look like,
>> > I even think being able to queue/dequeue an skb into different bpf maps
>> > should be the first thing to do here.  Looking forward to your patches.
>> 
>> Thanks! Guess I should go work on them, then :D
>
> Happy to review any RFCs.
>
>> 
>> >> - The behaviour is defined entirely by BPF program behaviour, and does
>> >>   not require setting up a qdisc hierarchy in addition to writing BPF
>> >>   code.
>> > Interesting idea.  If it does not need to use the qdisc object/interface
>> > and be able to do the qdisc hierarchy setup in a programmable way, it may
>> > be nice.  It will be useful for the future patches to come with some
>> > bpf prog examples to do that.
>> 
>> Absolutely; we plan to include example algorithm implementations as well!
>
> A weighted round robin queue setup might be a useful example and easy
> to implement/understand, but slightly more interesting than a pfifo. Also
> would force understanding multiple cpus and timer issues.

Yup, some sort of RR queueing is definitely on the list!

>> >> - It should be possible to structure the hooks in a way that allows
>> >>   reusing queueing algorithm implementations between the qdisc and XDP
>> >>   layers.
>> >> 
>> >> > You mention skb should not be exposed to userspace? Why? Whats the
>> >> > reason for this? Anyways we can make kernel only maps if we want or
>> >> > scrub the data before passing it to userspace. We do this already in
>> >> > some cases.
>> >> 
>> >> Yup, that's my approach as well.
>
> Having something reported back to userspace as the value might be helpful
> for debugging/tracing. Maybe the skb->hash? Then you could set this and
> then track a skb through the stack even when its in a bpf skb queue.

Yeah. I've just been using the pointer value for my initial testing.
That's not a good solution, of course, but having a visible identifier
would be neat. skb->hash makes sense for the qdisc layer, but not for
XDP...

>> >> 
>> >> > IMO it seems cleaner and more general to allow sk_buffs
>> >> > to be stored in maps and pulled back out later for enqueue/dequeue.
>> >> 
>> >> FWIW there's some gnarly details here (for instance, we need to make
>> >> sure the BPF program doesn't leak packet references after they are
>> >> dequeued from the map). My idea is to use a scheme similar to what we do
>> >> for XDP_REDIRECT, where a helper sets some hidden variables and doesn't
>> >> actually remove the packet from the queue until the BPF program exits
>> >> (so the kernel can make sure things are accounted correctly).
>> > The verifier is tracking the sk's references.  Can it be reused to
>> > track the skb's reference?
>> 
>> I was vaguely aware that it does this, but have not looked at the
>> details. Would be great if this was possible; will see how far I get
>> with it, and iterate from there (with your help, hopefully :))
>
> Also might need to drop any socket references from the networking side
> so an enqueued sock can't hold a socket open.

Not sure I'm following you here?

-Toke