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

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Date:   Thu, 14 Oct 2021 21:54:23 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        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

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.

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...

-Toke