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 [off-list ref] wrote:

Eric Dumazet [off-list ref] writes:

quoted

From: Eric Dumazet <edumazet@google.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.