Subject: Re: [RFC PATCH net-next 6/6] net_sched: codel: fix stale state for empty flows in fq_codel

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Wed, 18 Mar 2026 15:10:00 +0100

[email-masked] writes:

> From: Jonas Köppeler <[email-masked]>
>
> When codel_dequeue() finds an empty queue, it resets vars->dropping
> but does not reset vars->first_above_time.  The reference CoDel
> algorithm (Nichols & Jacobson, ACM Queue 2012) resets both:
>
>   dodeque_result codel_queue_t::dodeque(time_t now) {
>       ...
>       if (r.p == NULL) {
>           first_above_time = 0;   // <-- Linux omits this
>       }
>       ...
>   }
>
> Note that codel_should_drop() does reset first_above_time when called
> with a NULL skb, but codel_dequeue() returns early before ever calling
> codel_should_drop() in the empty-queue case.  The post-drop code paths
> do reach codel_should_drop(NULL) and correctly reset the timer, so a
> dropped packet breaks the cycle -- but the next delivered packet
> re-arms first_above_time and the cycle repeats.

Nice find! I think this is worth submitting as a separate fix, no?
Probably with a fixes tag:

Fixes: 76e3cc126bb2 ("codel: Controlled Delay AQM")

and possibly this one as well to aid stable backports:

Fixes: d068ca2ae2e6 ("codel: split into multiple files")

> For sparse flows such as ICMP ping (one packet every 200ms-1s), the
> first packet arms first_above_time, the flow goes empty, and the
> second packet arrives after the interval has elapsed and gets dropped.
> The pattern repeats, producing sustained loss on flows that are not
> actually congested.

Note that this only happens if the ping flow actually gets into a
dropping state before emptying out. Which rarely happens in practice,
which I'm guessing is why this was not discovered before now :)

> Test: veth pair, fq_codel, BQL disabled, 30000 iptables rules in the
> consumer namespace (NAPI-64 cycle ~14ms, well above fq_codel's 5ms
> target), ping at 5 pps under UDP flood:
>
>   Before fix:  26% ping packet loss
>   After fix:    0% ping packet loss
>
> Fix by resetting first_above_time to zero in the empty-queue path
> of codel_dequeue(), matching the reference algorithm.
>
> Fixes: 76e3cc126bb2 ("codel: Controlled Delay AQM")
> Co-developed-by: Jesper Dangaard Brouer <[email-masked]>
> Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> Signed-off-by: Jonas Köppeler <[email-masked]>
> Reported-by: Chris Arges <[email-masked]>
> Tested-by: Jonas Köppeler <[email-masked]>

Reviewed-by: Toke Høiland-Jørgensen <[email-masked]>