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

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Message-ID: <87a53xj5jv.fsf@toke.dk>
Date: Mon, 18 Aug 2025 12:19:16 +0200
From: Toke Høiland-Jørgensen <toke@...e.dk>
To: William Liu <will@...lsroot.io>, netdev@...r.kernel.org
Cc: 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 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.
>
> 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>
> ---
>  net/sched/sch_cake.c | 3 +++
>  1 file changed, 3 i
> nsertions(+)
>
> diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
> index dbcfb948c867..40814449f17a 100644
> --- a/net/sched/sch_cake.c
> +++ b/net/sched/sch_cake.c
> @@ -1934,6 +1934,9 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>  			cake_drop(sch, to_free);
>  		}
>  		b->drop_overlimit += dropped;
> +
> +		if (dropped)
> +			return NET_XMIT_CN;

cake_drop() may drop from a different flow, so we can't unconditionally
return NET_XMIT_CN. We'll have to check the return code of cake_drop()
and only return NET_XMIT_CN if it's the same flow we just enqueued a
packet to (which is also what fq_codel does, BTW).

-Toke