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