net/sched: Make cake_enqueue return NET_XMIT_CN when past buffer_limit

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: 046f6fd ("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>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20250819033601.579821-1-will@willsroot.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: BitsByWill

net/sched/sch_cake.c

@@ -1750,7 +1750,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	ktime_t now = ktime_get();
 	struct cake_tin_data *b;
 	struct cake_flow *flow;
-	u32 idx;
+	u32 idx, tin;
 
 	/* choose flow to insert into */
 	idx = cake_classify(sch, &b, skb, q->flow_mode, &ret);
@@ -1760,6 +1760,7 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		__qdisc_drop(skb, to_free);
 		return ret;
 	}
+	tin = (u32)(b - q->tins);
 	idx--;
 	flow = &b->flows[idx];
 
@@ -1927,13 +1928,22 @@ static s32 cake_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		q->buffer_max_used = q->buffer_used;
 
 	if (q->buffer_used > q->buffer_limit) {
+		bool same_flow = false;
 		u32 dropped = 0;
+		u32 drop_id;
 
 		while (q->buffer_used > q->buffer_limit) {
 			dropped++;
-			cake_drop(sch, to_free);
+			drop_id = cake_drop(sch, to_free);
+
+			if ((drop_id >> 16) == tin &&
+			    (drop_id & 0xFFFF) == idx)
+				same_flow = true;
 		}
 		b->drop_overlimit += dropped;
+
+		if (same_flow)
+			return NET_XMIT_CN;
 	}
 	return NET_XMIT_SUCCESS;
 }