Bad shaping at low rates, after commit 56b765b79 (htb: improved accuracy at high rates)

[Jesper Dangaard Brouer <brouer@redhat.com>]

Hi again,

I found another regression by commit 56b765b79 (htb: improved accuracy
at high rates).

After the commit HTB does not honor network rate limiting below 500kbps.

I have found that the bandwidth problem is related to GSO being enabled
on the device.  To test the situation, I had to use a real NIC (as I
were not allowed to disable GSO on dev "lo", test below).

Testing with 100kbit/s:
-----------------------
A localhost dev "lo" reproducer commands:
 tc qdisc add dev lo root handle 1: htb default 1
 tc class add dev lo parent 1: classid 1:1 htb rate 100kbit
 netserver
 netperf -t TCP_STREAM -l 60 -H 127.0.0.1 -- -m 1024

Measuring the qdisc "rate" via:
 tc -s -d class show dev lo classid 1:1

I know the qdisc "rate" is not not very reliable/accurate, but I do
see a large difference (and so do netperf reported throughput).
 - Kernel 2.6.32-352.el6 gives rate 98752bit
 - Kernel 3.10.0-rc1     gives rate 390208bit

My real-NIC test show:
 With GSO: rate 153248bit
 Without GSO: rate 99320bit

Now, we just have to figure out why commit 56b765b79e (htb: improved
accuracy at high rates) breaks bandwidth shaping at low rate with GSO
enabled...

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer

Data: real NIC
==============

With GSO: rate 153248bit
---------
class htb 1:1 root prio 0 quantum 1250 rate 100000bit ceil 100000bit
burst 1600b/1 mpu 0b overhead 0b cburst 1600b/1 mpu 0b overhead 0b
level 0 Sent 844892 bytes 568 pkt (dropped 0, overlimits 0 requeues 0)
rate 153248bit 13pps backlog 0b 7p requeues 0 lended: 87 borrowed: 0
giants: 0 tokens: -847776554 ctokens: -847776554

Without GSO: rate 99320bit
------------
(disabled via: ethtool -K eth63 gso off gro off tso off)

class htb 1:1 root prio 0 quantum 1250 rate 100000bit ceil 100000bit
burst 1600b/1 mpu 0b overhead 0b cburst 1600b/1 mpu 0b overhead 0b
level 0 Sent 1706968 bytes 1164 pkt (dropped 0, overlimits 0 requeues
0) rate 99320bit 8pps backlog 0b 51p requeues 0 lended: 664 borrowed: 0
giants: 0 tokens: -121054491 ctokens: -121054491