net_sched: sch_fq: fix pacing delay underflow with pacing offload

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When pacing offload is enabled (q->offload_horizon > 0),
FQ can dequeue packets early (now < f->time_next_packet).

In this case, the drift calculation (now - f->time_next_packet)
underflows to a large unsigned value.

min(len/2, now - f->time_next_packet) then evaluates to len/2,
incorrectly halving the pacing delay for the next packet.

Fix this by only applying drift compensation if now > f->time_next_packet.

This bug was triggered when flow_max_rate was set on the qdisc
or for non EDT packets (packets with a zero skb->tstamp).

Fixes: f26080d470 ("net_sched: sch_fq: add the ability to offload pacing")
Reported-by: Willem de Bruijn <willemb@google.com>
Closes: https://lore.kernel.org/netdev/CANn89iK6O7ujR9zCJzd04MNLQoDi3mA+HWsR-hgQWYzLS3gZfw@mail.gmail.com/
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260820120706.1995449-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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Eric Dumazet 2026-08-20 08:06:25 -04:00 • committed by Jakub Kicinski

parent 039f248a6c

commit 137b8ae233

1 changed files with 6 additions and 2 deletions

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net/sched/sch_fq.c

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