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

[John Fastabend <john.fastabend@gmail.com>]

Toke Høiland-Jørgensen wrote:
> Martin KaFai Lau <kafai@fb.com> writes:
> 
> > On Wed, Sep 01, 2021 at 12:42:03PM +0200, Toke Høiland-Jørgensen wrote:
> >> John Fastabend <john.fastabend@gmail.com> writes:
> >> 
> >> > Cong Wang wrote:
> >> >> On Tue, Aug 24, 2021 at 4:47 PM Martin KaFai Lau <kafai@fb.com> 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.

> 
> 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 ;)

> 
> >> > 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.

> 
> >> - 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.

> >> 
> >> > 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.

> 
> -Toke
>