netdev - Re: [PATCH net-next 2/2] fq_codel: implement L4S style ce_threshold

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Date:   Thu, 14 Oct 2021 14:35:24 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Ingemar Johansson S <ingemar.s.johansson@...csson.com>,
        Tom Henderson <tomh@...h.org>, Bob Briscoe <in@...briscoe.net>
Subject: Re: [PATCH net-next 2/2] fq_codel: implement L4S style
 ce_threshold_ect1 marking

On Thu, Oct 14, 2021 at 12:54 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Eric Dumazet <eric.dumazet@...il.com> writes:
>
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > Add TCA_FQ_CODEL_CE_THRESHOLD_ECT1 boolean option to select Low Latency,
> > Low Loss, Scalable Throughput (L4S) style marking, along with ce_threshold.
> >
> > If enabled, only packets with ECT(1) can be transformed to CE
> > if their sojourn time is above the ce_threshold.
> >
> > Note that this new option does not change rules for codel law.
> > In particular, if TCA_FQ_CODEL_ECN is left enabled (this is
> > the default when fq_codel qdisc is created), ECT(0) packets can
> > still get CE if codel law (as governed by limit/target) decides so.
>
> The ability to have certain packets receive a shallow marking threshold
> and others regular ECN semantics is no doubt useful. However, given that
> it is by no means certain how the L4S experiment will pan out (and I for
> one remain sceptical that the real-world benefits will turn out to match
> the tech demos), I think it's premature to bake the ECT(1) semantics
> into UAPI.

Chicken and egg problem.
We had fq_codel in linux kernel years before RFC after all :)

>
> So how about tying this behaviour to a configurable skb->mark instead?
> That way users can get the shallow marking behaviour for any subset of
> packets they want, simply by installing a suitable filter on the
> qdisc...

This seems an idea, but do you really expect users installing a sophisticated
filter ? Please provide more details, and cost analysis.
(Having to install a filter is probably more expensive than testing a boolean,
after the sojourn time  has exceeded the threshold)

Given that INET_ECN_set_ce(skb) only operates on ECT(1) and ECT(0),
I guess we could  use a bitmask of two bits so that users can decide
which code points can become CE.