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virnetdevopenvswitch: Warn on unsupported QoS settings

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Let me preface this with stating the obvious: documentation on
QoS in OVS is very sparse. This is all based on my observation
and OVS codebase analysis.

For the following QoS setting:

  <bandwidth>
    <inbound average="512" peak="1024" burst="32"/>
  </bandwidth>

the following QoS setting is generated into OVS (NB, our XML
values are in KiB/s, OVS has them in bits/s):

  # ovs-vsctl list qos
  _uuid               : a087226b-2da6-4575-ad4c-bf570cb812a9
  external_ids        : {ifname=vnet1, vm-id="7714e6b5-4885-4140-bc59-2f77cc99b3b5"}
  other_config        : {burst="262144", max-rate="8192000", min-rate="4096000"}
  queues              : {0=655bf3a7-e530-4516-9caf-ec9555dfbd4c}
  type                : linux-htb

from which the following topology is generated:

  # for i in qdisc class; do tc -s -d -g $i show dev vnet1; done
  qdisc htb 1: root refcnt 2 r2q 10 default 0x1 direct_packets_stat 0 ver 3.17 direct_qlen 1000
   Sent 2186 bytes 16 pkt (dropped 0, overlimits 0 requeues 0)
   backlog 0b 0p requeues 0

  +---(1:fffe) htb rate 8192Kbit ceil 8192Kbit linklayer ethernet burst 1499b/1mpu 60b cburst 1499b/1mpu 60b level 7
       |       Sent 2186 bytes 16 pkt (dropped 0, overlimits 0 requeues 0)
       |       backlog 0b 0p requeues 0
       |
       +---(1:1) htb prio 0 quantum 51200 rate 4096Kbit ceil 8192Kbit linklayer ethernet burst 32Kb/1mpu 60b cburst 32Kb/1mpu 60b level 0
                 Sent 2186 bytes 16 pkt (dropped 0, overlimits 0 requeues 0)
                 backlog 0b 0p requeues 0

Long story short, the default class (1:) for an OVS interface has
average and peak set exactly as requested. But since it's nested
under another class (1:fffe), it can borrow unused bandwidth. And
the parent is set to have rate = ceil = peak from our XML. From
[1]: htb_tc_install() calls htb_parse_qdisc_details__() which
sets: 'hc->min_rate = hc->max_rate;' and then calls
htb_setup_class_(..., tc_make_handle(1, 0xfffe), tc_make_handle(1, 0), &hc);
to set up the top parent class.

In other words - the interface is set up to so that it can always
consume 'peak' bandwidth and there is no way for us to set it up
differently. It's too late to deny setting 'peak' different to
'average' at XML validation phase so do the next best thing -
throw a warning, just like we do in case <bandwidth/> is set for
an unsupported <interface/> type.

1: https://github.com/openvswitch/ovs/blob/main/lib/netdev-linux.c#L5039
Resolves: https://issues.redhat.com/browse/RHEL-53963
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>
This commit is contained in:
Michal Privoznik 2024-10-22 09:11:33 +02:00
parent 9132b486a3
commit a3b8753db9
1 changed files with 8 additions and 0 deletions
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8
src/util/virnetdevopenvswitch.c
View File

@ -945,6 +945,10 @@ virNetDevOpenvswitchInterfaceSetQos(const char *ifname,
}
if (tx && tx->average) {
if (tx->peak && tx->peak != tx->average) {
VIR_WARN("Setting different 'peak' value than 'average' for QoS for OVS interface %s might have unexpected results",
ifname);
}
if (virNetDevOpenvswitchInterfaceSetTxQos(ifname, tx, vmuuid) < 0)
return -1;
} else {
@ -954,6 +958,10 @@ virNetDevOpenvswitchInterfaceSetQos(const char *ifname,
}
if (rx) {
if (rx->peak && tx->peak != rx->average) {
VIR_WARN("Setting different 'peak' value than 'average' for QoS for OVS interface %s might have unexpected results",
ifname);
}
if (virNetDevOpenvswitchInterfaceSetRxQos(ifname, rx) < 0)
return -1;
} else {