netdev - Re: [RFC Patch net-next v2] net_sched: introduce eBPF based Qdisc

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Date:   Wed, 22 Sep 2021 18:49:02 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Network Development <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 v2] net_sched: introduce eBPF based Qdisc

On Tue, Sep 21, 2021 at 9:13 PM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> "This *incomplete* patch introduces a programmable Qdisc with
> eBPF.  The goal is to make this Qdisc as programmable as possible,
> that is, to replace as many existing Qdisc's as we can, no matter
> in tree or out of tree. And we want to make programmer's and researcher's
> life as easy as possible, so that they don't have to write a complete
> Qdisc kernel module just to experiment some queuing theory."

The inspiration was clear and obvious.
Folks in the thread had the same idea long before your patches came out.
But it doesn't matter who came up with an idea to implement qdisc in bpf first.
Everyone in the thread agreed that such a feature would be great to have.
The point people explicitly highlighted is that qdisc is not only about skbs.
The queuing concepts (fq, codel, etc) are useful in xdp and non networking
contexts too.
That's why approaching the goal with more generic ambitions was requested.

> If you compare it with V1, V2 explains the use case in more details,
> which is to target Qdisc writers, not any other. Therefore, the argument
> of making it out of Qdisc is non-sense, anything outside of Qdisc is
> not even my target. Of course you can do anything in XDP, but it has
> absolutely nothing with my goal here: Qdisc.

Applying queuing discipline to non-skb context may be not your target
but it's a reasonable and practical request to have. The kernel is not
about solving one person's itch. The kernel has to be optimized for
all use cases.

>
> I also addressed the skb map concern:
>
> " 2b) Kernel would lose the visibility of the "queues", as maps are only
>    shared between eBPF programs and user-space. These queues still have to
>    interact with the kernel, for example, kernel wants to reset all queues
>    when we reset the network interface, kernel wants to adjust number of
>    queues if they are mapped to hardware queues."
>
> More than writing, I even tried to write a skb map by myself,

Cool and it sounds that you've failed to do so?
At the same time Toke mentioned that he has a prototype of suck skb map.
What addition data can open your mind that skb/packet map is more
universal building block for queuing discipline in different layers?