Eric Dumazet [off-list ref] wrote:

quoted

diff --git a/net/sched/sch_hfsc.c b/net/sched/sch_hfsc.c
index d783d7c..0854be3 100644
--- a/net/sched/sch_hfsc.c
+++ b/net/sched/sch_hfsc.c

@@ -1583,6 +1583,7 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 {
 	struct hfsc_class *cl;
 	int uninitialized_var(err);
+	unsigned int qlen;
 
 	cl = hfsc_classify(skb, sch, &err);
 	if (cl == NULL) {

@@ -1592,6 +1593,7 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 		return err;
 	}
 
+	qlen = cl->qdisc->q.qlen;
 	err = qdisc_enqueue(skb, cl->qdisc);
 	if (unlikely(err != NET_XMIT_SUCCESS)) {
 		if (net_xmit_drop_count(err)) {

@@ -1601,7 +1603,7 @@ hfsc_enqueue(struct sk_buff *skb, struct Qdisc *sch)
 		return err;
 	}
 
-	if (cl->qdisc->q.qlen == 1)
+	if (qlen == 0)
 		set_active(cl, qdisc_pkt_len(skb));
 
 	sch->q.qlen++;

Well, I am not sure HFSC can deal with non work conserving qdisc
anyway ?

As long as child qdisc dequeue() returns an skb if qlen > 0 it should
work.

Call to set_active(cl, qdisc_pkt_len(skb)); would tell you that HFSC
does not expect another packet than @skb being the next dequeued one.

Good point.  I'll look into it in more detail but AFAIU this "only"
means we might overshoot a defined realtime/deadline criterion,
I don't (yet) see how it could explain Miroslavs bug report.

If you want to make HFSC generic, you would need a lot more changes.

Could you elaborate?

I'm not interested in e.g. tbf leaves (makes no sense to me), but I
think it should play nice with netem, sfq, codel, fq_codel, fq, ...

I don't see any problems with fq_codel + hfsc unless I deliberately
misconfigure fq_codel (setting extremely low mem limit, e.g. 32kbyte).