netdev - knob to disable locally-originating qdisc optimisation?

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Date:   Wed, 26 Apr 2023 14:54:30 +0200 (CEST)
From:   Thorsten Glaser <t.glaser@...ent.de>
To:     netdev@...r.kernel.org
cc:     Haye.Haehne@...ekom.de
Subject: knob to disable locally-originating qdisc optimisation?

Hi,

when traffic (e.g. iperf) is originating locally (as opposed to
forward traffic), the Linux kernel seems to apply some optimisations
probably to reduce overall bufferbloat: when the qdisc is “full” or
(and especially) when its dequeue often returns NULL (because packets
are delayed), the sender traffic rate is reduced by as much as ⅓ with
40 ms extra latency (30 → 20 Mbit/s).

This is probably good in general but not so good for L4S where we
actually want the packets to queue up in the qdisc so they get ECN
marking appropriately (I guess there probably are some socket ioctls
or something with which the sending application could detect this
state; if so, we’d be interested in knowing about them as well).

This is especially bad in a testbed for writing L4S-aware applications,
so if there’s a knob (sysctl or something) to disable this optimisation
please do tell (I guess probably not, but asking doesn’t hurt).

Thanks,
//mirabilos
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