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: <CAM0EoMk60U8VV_dqiuXsE+AWQHAoiFKSc4FfBgVSJs1sWp+B5w@mail.gmail.com>
Date: Wed, 20 Aug 2025 02:45:28 -0400
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Toke Høiland-Jørgensen <toke@...e.dk>
Cc: William Liu <will@...lsroot.io>, netdev@...r.kernel.org, dave.taht@...il.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
Subject: Re: [PATCH net v2 1/2] net/sched: Make cake_enqueue return
 NET_XMIT_CN when past buffer_limit

On Tue, Aug 19, 2025 at 4:51 AM Toke Høiland-Jørgensen <toke@...e.dk> wrote:
>
> 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>

Reviewed-by: Jamal Hadi Salim <jhs@...atatu.com>

cheers,
jamal
>