[Cake] Re: [PATCH net v2 1/2] net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit

["Toke Høiland-Jørgensen" <toke@toke.dk>]

William Liu <will@willsroot.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@willsroot.io>
> Reviewed-by: Savino Dicanosa <savy@syst3mfailure.io>

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