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

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Message-ID: <CAM_iQpUSJ=QRdjwo-0DG4O1MRkhu8k1yQQx2KM_UhPp=bkeOeA@mail.gmail.com>
Date:   Fri, 3 Sep 2021 18:05:55 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Martin KaFai Lau <kafai@...com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        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

On Tue, Aug 31, 2021 at 10:45 PM John Fastabend
<john.fastabend@...il.com> wrote:
>
> Cong Wang wrote:
> > On Tue, Aug 24, 2021 at 4:47 PM Martin KaFai Lau <kafai@...com> 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).
> >
> > Thanks.
>
> 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.

Because it is pointless to expose them to user-space in this context.
For example, what is the point of dropping a packet from user-space
in the context of Qdisc?

And I don't think there is a way for user-space to read those skb's inside
a map, which makes it more pointless.

>
> 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.

I am not aware of any kernel-only map. For starters, we can't create
a map from kernel-space.

>
> IMO it seems cleaner and more general to allow sk_buffs
> to be stored in maps and pulled back out later for enqueue/dequeue.

Which exact map are you referring to? The queue map? It would only
provide FIFO. We want to give users as much freedom to order the
skbs as we can, I doubt hashmap could offer such freedom.

>
> I think one trick might be how to trigger the dequeue event on
> transition from stopped to running net_device or other events like
> this, but that could be solved with another program attached to
> those events to kick the dequeue logic.

I think we can still use current enqueue/dequeue eBPF program
except we need to transfer skb ownership and storage to map.

Thanks.