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: 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>
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>

diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c
index dbcfb948c86707a4e29d3445524e1fda4f8fd617..32bacfc314c260dccf94178d309ccb2be22d69e4 100644 (file)
--- a/net/sched/sch_cake.c
+++ b/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;
 }