[Bloat] Inaccurate rates with HTB/HFSC+fq_codel on router due to VLAN?

[xnor <xnoreq@gmail.com>]

Hello,

I have a tl-wdr3600 router with openwrt (linux 3.18, config at [1]) and 
want to shape ingress traffic.

The WAN port, interface eth0.2, is connected to a cable modem.
I use the following script:

tc qdisc add dev eth0.2 handle ffff: ingress
ifconfig ifb0 up txqueuelen 1000
tc qdisc add dev ifb0 root        handle 1: htb default 10
tc class add dev ifb0 parent 1: classid 1:1 htb rate 18000kbit
tc class add dev ifb0 parent 1:1 classid 1:10 htb rate 18000kbit ceil 
18000kbit
tc filter add dev eth0.2 parent ffff: protocol all prio 1 u32 match u32 
0 0 action mirred egress redirect dev ifb0
tc qdisc add dev ifb0 parent 1:10 handle 100: fq_codel limit 1000 
quantum 1514 noecn

Now the problem is that it is very inaccurate, and this inaccuracy 
increases with number of TCP download connections.
With HTB (it's worse with HFSC) and 20 connections (rates calculated 
with /sys/class/net/eth0.2/statistics/rx_bytes over 5 seconds each):
19012.610 kbit
19266.877 kbit
19303.923 kbit

With a single connection and HTB, I only get about 17.2 Mbit. Curiously, 
/sys/class/net/ifb0/statistics show about 18.1 +/- 0.1 Mbit

So I did a tcpdump of eth0.2 and ifb0:
eth0.2's largest frames are 1514 bytes ethernet type IP.
ifb0's are 1518 bytes (including the VLAN tag I guess) with some unknown 
type.


What do I need to change to make shaping with ifb accurate? Ideally with 
both HTB and HFSC.


Additional question: What's the effect of the system not having HR 
timers?


[1] 
https://github.com/openwrt/openwrt/tree/chaos_calmer/target/linux/ar71xx