Why does QoS CBQ queue priority not take effect

Hi
I tried to test the priotity feature of the QoS CBQ queue on the chip of MT7621 (openwrt system version: 18.06), and found that the priority does not take effect. The test on centos is no problem, who can help me, thanks.

The test method is as follows:
The uplink bandwidth of the wan interface of the MT7621 is limited to 10mbit. The lan zone of the MT7621 is connected to the A/B/C three terminals. Each terminal runs iperf client to send 10mbit/s test data. On the MT7621,i used TC to configure the CBQ as three terminals. The traffic class is classified, and the class priority is set to A:3, B:5, and C:7. In principle, mt7621 preferentially forwards the data of the A terminal prio 3, and the vacant bandwidth is forwarded to the B/C data. However, in fact, the iperf server receives the message in the case of A/B/C 3 terminals, and the bandwidth rate of each terminal is about 3.2 mbit/s, and the data of A is not preferentially forwarded according to the priority, and there is free bandwidth. Then forward the data of the B/C terminal.
I use iptables to add different mark tags to the three terminal assignments, and use the tc filter to match the mark tags to the tc class classification of the A/BC terminal respectively; see the class's normal forwarding A/B/C terminal traffic.

root@Ti-C1100E:~# tc class show dev ifb0
class cbq 1: root rate 100Mbit (bounded,isolated) prio no-transmit
class cbq 1:1 parent 1: rate 100Mbit (bounded) prio 1
class cbq 1:10 parent 1:1 leaf 100: rate 10Mbit (bounded) prio 3    **#A terminal class,prio 3**
class cbq 1:20 parent 1:1 leaf 200: rate 10Mbit (bounded) prio 5    **#B terminal class,prio 5**
class cbq 1:30 parent 1:1 leaf 300: rate 10Mbit (bounded) prio 7    **#C terminal class,prio 7**
class cbq 1:5 parent 1:1 leaf 50: rate 10Mbit (bounded) prio 1
class fq_codel 50:29a parent 50: 

root@Ti-C1100E:~# tc -s class show dev ifb0
class cbq 1: root rate 100Mbit (bounded,isolated) prio no-transmit
 Sent 3545019555 bytes 2380654 pkt (dropped 0, overlimits 0 requeues 0) 
 backlog 0b 0p requeues 0
  borrowed 0 overactions 0 avgidle 1250 undertime 0
class cbq 1:1 parent 1: rate 100Mbit (bounded) prio 1
 Sent 3542718030 bytes 2356329 pkt (dropped 0, overlimits 0 requeues 0) 
 backlog 0b 0p requeues 0
  borrowed 0 overactions 0 avgidle 10234 undertime 0
class cbq 1:10 parent 1:1 leaf 100: rate 10Mbit (bounded) prio 3      **#A terminal class test data**
 Sent 1173944016 bytes 776418 pkt (dropped 0, overlimits 4474038 requeues 0) 
 backlog 0b 0p requeues 0
  borrowed 0 overactions 491227 avgidle 102390 undertime 0
class cbq 1:20 parent 1:1 leaf 200: rate 10Mbit (bounded) prio 5       **#B terminal class test data**
 Sent 1181780712 bytes 781601 pkt (dropped 0, overlimits 3717156 requeues 0) 
 backlog 0b 0p requeues 0
  borrowed 0 overactions 492032 avgidle 102390 undertime 0
class cbq 1:30 parent 1:1 leaf 300: rate 10Mbit (bounded) prio 7       **#C terminal class test data**
 Sent 1185932664 bytes 784347 pkt (dropped 0, overlimits 2939771 requeues 0) 
 backlog 0b 0p requeues 0
  borrowed 0 overactions 493095 avgidle 59064 undertime 0


**CBQ Configuration:**
tc qdisc add dev ifb0 root handle 1:30 cbq bandwidth 100Mbit avpkt 1000 cell 8 mpu 64
tc class add  dev ifb0 parent 1:0 classid 1:1 cbq bandwidth 100Mbit rate 100Mbit weight  1Mbit prio 1 allot 1514 cell 8 maxburst 20 avpkt 8192 mpu 64 bounded
tc class add  dev ifb0 parent 1:1 classid 1:10 cbq bandwidth 10Mbit rate 10Mbit weight  1Mbit prio 3 allot 1514 cell 8 maxburst 20 avpkt 8192 mpu 64 bounded
tc class add  dev ifb0 parent 1:1 classid 1:20 cbq bandwidth 10Mbit rate 10Mbit weight  1Mbit prio 5 allot 1514 cell 8 maxburst 20 avpkt 8192 mpu 64 bounded
tc class add  dev ifb0 parent 1:1 classid 1:30 cbq bandwidth 10Mbit rate 10Mbit weight  1Mbit prio 7 allot 1514 cell 8 maxburst 20 avpkt 8192 mpu 64 bounded

tc filter add dev ifb0 parent 1: protocol ip prio 1 handle 100 fw classid 1:10
tc filter add dev ifb0 parent 1: protocol ip prio 3 handle 200 fw classid 1:20
tc filter add dev ifb0 parent 1: protocol ip prio 500 handle 300 fw classid 1:30