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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 02 Sep 2021 18:57:17 +0200

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?

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? :)

>> > 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

>> - 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!

>> - 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.
>> 
>> > 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 :))

-Toke