netdev - Re: inter-qdisc communication?

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Message-ID: <20220826180641.1e856c1d@kernel.org>
Date:   Fri, 26 Aug 2022 18:06:41 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Thorsten Glaser <t.glaser@...ent.de>
Cc:     netdev@...r.kernel.org
Subject: Re: inter-qdisc communication?

On Sat, 27 Aug 2022 02:45:02 +0200 (CEST) Thorsten Glaser wrote:
> > How do you add latency on ingress? 🤔 
> > The ingress qdisc is just a stub to hook classifiers/actions.   
> 
> Oh, damn. Then, I guess, I’ll have to do that on egress on
> the other interface, which makes it at least symmetric for
> passing traffic but catch not the set of traffic it should.
> Especially not the traffic terminating locally. Meh.
> 
> The question remains the same, just the use case magically
> mutated under me.
> 
> (Maybe if there were documentation like an intro to qdisc
> writing like I asked for already, I’d have known that.)

These days the recommendation for adding workload specific sauce 
at the qdisc layer is to use the fq qdisc (mq + fq if you have 
a multi queue device) and add a BPF program on top (cls-bpf)
which sets transmission times for each packet.

Obviously you can still write a qdisc if you wish or your needs 
are sufficiently hardcore.

On the docs, nothing official AFAIK, if it doesn't pop up in 
the first two pages of Google results it probably doesn't exist :(