netdev - Re: [RFC PATCH net-next 0/3] TC: Introduce qevents

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Date:   Tue, 26 May 2020 21:09:03 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Petr Machata <petrm@...lanox.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Ido Schimmel <idosch@...lanox.com>
Subject: Re: [RFC PATCH net-next 0/3] TC: Introduce qevents

On Tue, May 26, 2020 at 10:11 AM Petr Machata <petrm@...lanox.com> wrote:
>
> The Spectrum hardware allows execution of one of several actions as a
> result of queue management events: tail-dropping, early-dropping, marking a
> packet, or passing a configured latency threshold or buffer size. Such
> packets can be mirrored, trapped, or sampled.
>
> Modeling the action to be taken as simply a TC action is very attractive,
> but it is not obvious where to put these actions. At least with ECN marking
> one could imagine a tree of qdiscs and classifiers that effectively
> accomplishes this task, albeit in an impractically complex manner. But
> there is just no way to match on dropped-ness of a packet, let alone
> dropped-ness due to a particular reason.
>
> To allow configuring user-defined actions as a result of inner workings of
> a qdisc, this patch set introduces a concept of qevents. Those are attach
> points for TC blocks, where filters can be put that are executed as the
> packet hits well-defined points in the qdisc algorithms. The attached
> blocks can be shared, in a manner similar to clsact ingress and egress
> blocks, arbitrary classifiers with arbitrary actions can be put on them,
> etc.

This concept does not fit well into qdisc, essentially you still want to
install filters (and actions) somewhere on qdisc, but currently all filters
are executed at enqueue, basically you want to execute them at other
pre-defined locations too, for example early drop.

So, perhaps adding a "position" in tc filter is better? Something like:

tc qdisc add dev x root handle 1: ... # same as before
tc filter add dev x parent 1:0 position early_drop matchall action....

And obviously default position must be "enqueue". Makes sense?

(The word "position" may be not accurate, but hope you get my point
here.)

Thanks.