netdev - Re: [PATCH net v2 1/2] net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer

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Message-ID: <871pp7k82y.fsf@toke.dk>
Date: Tue, 19 Aug 2025 10:51:33 +0200
From: Toke Høiland-Jørgensen <toke@...e.dk>
To: William Liu <will@...lsroot.io>, netdev@...r.kernel.org
Cc: dave.taht@...il.com, jhs@...atatu.com, xiyou.wangcong@...il.com,
 pabeni@...hat.com, kuba@...nel.org, savy@...t3mfailure.io,
 jiri@...nulli.us, davem@...emloft.net, edumazet@...gle.com,
 horms@...nel.org, cake@...ts.bufferbloat.net, William Liu
 <will@...lsroot.io>
Subject: Re: [PATCH net v2 1/2] net/sched: Make cake_enqueue return
 NET_XMIT_CN when past buffer_limit

William Liu <will@...lsroot.io> writes:

> The following setup can trigger a WARNING in htb_activate due to
> the condition: !cl->leaf.q->q.qlen
>
> tc qdisc del dev lo root
> tc qdisc add dev lo root handle 1: htb default 1
> tc class add dev lo parent 1: classid 1:1 \
>        htb rate 64bit
> tc qdisc add dev lo parent 1:1 handle f: \
>        cake memlimit 1b
> ping -I lo -f -c1 -s64 -W0.001 127.0.0.1
>
> This is because the low memlimit leads to a low buffer_limit, which
> causes packet dropping. However, cake_enqueue still returns
> NET_XMIT_SUCCESS, causing htb_enqueue to call htb_activate with an
> empty child qdisc. We should return NET_XMIT_CN when packets are
> dropped from the same tin and flow.
>
> I do not believe return value of NET_XMIT_CN is necessary for packet
> drops in the case of ack filtering, as that is meant to optimize
> performance, not to signal congestion.
>
> Fixes: 046f6fd5daef ("sched: Add Common Applications Kept Enhanced (cake) qdisc")
> Signed-off-by: William Liu <will@...lsroot.io>
> Reviewed-by: Savino Dicanosa <savy@...t3mfailure.io>

Acked-by: Toke Høiland-Jørgensen <toke@...e.dk>