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? 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
