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 Reviewed-by: Savino Dicanosa --- v1 -> v2: only return NET_XMIT_CN when dropping from the same tin and flow --- net/sched/sch_cake.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/net/sched/sch_cake.c b/net/sched/sch_cake.c index dbcfb948c867..32bacfc314c2 100644 --- 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; } -- 2.43.0