net/sched: fq_codel: match the no-drop threshold to the packet size

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commit d9ebd8f9aa ("net/sched: fq_codel: clamp default quantum and mtu")
clamped both q->quantum and q->cparams.mtu to [256, FQ_CODEL_QUANTUM_MAX].
The two fields mean different things: quantum is a DRR credit that wants
the 256 floor, but cparams.mtu is the CoDel no-drop threshold
(codel_impl.h "*backlog <= params->mtu"). On a link whose maximum
transmitted packet size is below 256, the floor extends CoDel's
minimum-backlog exemption beyond one packet and delays drop or mark
eligibility by several small packets.

Split the clamp. quantum keeps [256, FQ_CODEL_QUANTUM_MAX]; cparams.mtu
tracks psched_mtu() (the device MTU plus its hard-header length) with
only the upper bound that guards the original overflow (psched_mtu()
wrapping to ~2 GiB on a huge-MTU device).

Conditions to recreate the bug: attach an fq_codel qdisc on a link
whose MTU plus hard_header_len is below 256 (e.g. a CAN interface). At
that MTU the no-drop threshold must equal the device MTU plus its
hard-header length; before this patch it was forced to 256.
Basic Testing done: with dev->mtu=100 and hard_header_len=14, a
return probe on fq_codel_init() observed cparams.mtu change from 256 to 114

Fixes: d9ebd8f9aa ("net/sched: fq_codel: clamp default quantum and mtu")
Reported-by: Sashiko (nipa) <sashiko-bot@kernel.org>
Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260819143136.57350-1-jhs@mojatatu.com
Signed-off-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/QDISC-34MS.v1.20260925165535@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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This commit is contained in:

Jamal Hadi Salim 2026-09-26 14:03:27 -04:00 • committed by Paolo Abeni

parent 0923198be4

commit d031465acc

1 changed files with 2 additions and 2 deletions

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net/sched/sch_fq_codel.c

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