Martin KaFai Lau [off-list ref] writes:

On Fri, Sep 03, 2021 at 04:44:04PM +0200, Toke Høiland-Jørgensen wrote:

quoted

Martin KaFai Lau [off-list ref] writes:

quoted

On Fri, Sep 03, 2021 at 12:27:52AM +0200, Toke Høiland-Jørgensen wrote:

quoted

quoted

quoted

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?

Instead of bpftool can only introspect bpf qdisc and the existing tc
can only introspect kernel qdisc,  it will be nice to have bpf
qdisc work as other qdisc and showing details together with others
in tc.  e.g. a bpf qdisc export its data/stats with its btf-id
to tc and have tc print it out in a generic way?

I'm not opposed to the idea, certainly. I just wonder if people who go
to the trouble of writing a custom qdisc in BPF will feel it's worth it
to do the extra work to make this available via a second API. We could
certainly encourage it, and some things are easy (drop and pkt counters,
etc), but other things (like class stats) will depend on the semantics
of the qdisc being implemented, so will require extra work from the BPF
qdisc developer...

Right, different qdisc has different stats, I think it is currently
stored in qdisc_priv()?  When a qdisc is created, a separate priv is
created together.

Yes, the bpf qdisc prog can store its stats to a bpf map, but then
when the same prog attached to different qdiscs, it has to create
different stats maps?

Hmm, yeah, I guess it would. But if it's storing the packets in a map it
would need to have separate instances of those as well. I was kinda
assuming that a separate instance of the BPF program would be loaded
into the kernel for each qdisc instance, with its own instance of all
maps etc.

Also, instead of ->enqueue() itself is a bpf prog, having an
->enqueue() preparing a bpf ctx (zeroing, assigning...etc) and then
make another call to a bpf prog will all add some costs.

Hmm, yeah, I guess, but I kinda doubt we can avoid having *some* kind of
setup to get the right semantics for the BPF program, which might as
well be in the qdisc enqueue() func. But let's see, happy to be proved
wrong on this :)

That said, I still think it needs a bpf skb map that can queue/dequeue
skb first.  Then it will become possible to prototype different interface
ideas.

Agreed!

-Toke