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

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Date:   Fri, 10 Sep 2021 13:31:01 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Cong Wang <cong.wang@...edance.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: Re: [RFC Patch net-next] net_sched: introduce eBPF based Qdisc

Martin KaFai Lau <kafai@...com> writes:

> On Fri, Sep 03, 2021 at 04:44:04PM +0200, Toke Høiland-Jørgensen wrote:
>> Martin KaFai Lau <kafai@...com> writes:
>> 
>> > On Fri, Sep 03, 2021 at 12:27:52AM +0200, Toke Høiland-Jørgensen wrote:
>> >> >> 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