kern/56319: CBQ: high bandwidths are inaccurate

[ozaki-r%netbsd.org@localhost]

>Number:         56319
>Category:       kern
>Synopsis:       CBQ: high bandwidths are inaccurate
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jul 21 05:15:00 +0000 2021
>Originator:     Ryota Ozaki
>Release:        current
>Organization:
>Environment:
NetBSD kvm 9.99.86 NetBSD 9.99.86 (KVM) #83: Wed Jul 21 12:14:19 JST 2021  (hidden) amd64
>Description:
For example even if you specify 750 Mbps to a class, it is treated as 800 Mbps
internally.

This happens because CBQ represents a bandwidth as nanosecond per byte
and an integer in the kernel.  So bandwidths from 730 Mbps to 800 Mbps
are all rounded to 800 Mbps. 

>How-To-Repeat:
# cat /etc/altq.conf
interface vioif1 bandwidth 1000M cbq no-control
class cbq vioif1 root NULL pbandwidth 100
class cbq vioif1 0: root pbandwidth 75 default
# altqd
# altqstat -c 1
altqstat: cbq on interface vioif1
Class 1 on Interface vioif1: root
        priority: 0 depth: 1 offtime: 1 [us] wrr_allot: 1500 bytes
        nsPerByte: 8    (1000.00Mbps),  Measured: 90711.30M [bps]
        pkts: 0,        bytes: 0
        overs: 0,       overactions: 0
        borrows: 0,     delays: 0
        drops: 0,       drop_bytes: 0
        QCount: 0,      (qmax: 30)
        AvgIdle: 0 [us],        (maxidle: 0 minidle: -12 [us])
Class 2 on Interface vioif1: 0:
        priority: 1 depth: 0 offtime: 7 [us] wrr_allot: 1500 bytes
        nsPerByte: 10   (800.00Mbps)
        pkts: 0,        bytes: 0
        overs: 0,       overactions: 0
        borrows: 0,     delays: 0
        drops: 0,       drop_bytes: 0
        QCount: 0,      (qmax: 30)
        AvgIdle: 2 [us],        (maxidle: 2 minidle: -15 [us])

>Fix:
Treat bandwidths as picosecond per byte in the kernel.