[Bug 1999814] Re: [SRU] Allow for specifying common baseline CPU model with disabled feature
Mauricio Faria de Oliveira
1999814 at bugs.launchpad.net
Fri Jul 26 20:52:24 UTC 2024
Uploaded a combined SRU for Jammy (ubuntu2.7) with
bug 1999814 (previously in jammy-proposed; ubuntu2.4) and
bug 2024258 (previously in jammy's unapproved queue; ubuntu2.5),
which were superseded by security upload (jammy-security; ubuntu2.6).
Build-tested (with build-time unit tests) in a PPA.
** Changed in: nova (Ubuntu Jammy)
Status: Fix Committed => In Progress
** Tags removed: verification-needed verification-needed-jammy
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https://bugs.launchpad.net/bugs/1999814
Title:
[SRU] Allow for specifying common baseline CPU model with disabled
feature
Status in OpenStack Compute (nova):
Expired
Status in OpenStack Compute (nova) ussuri series:
New
Status in OpenStack Compute (nova) victoria series:
Won't Fix
Status in OpenStack Compute (nova) wallaby series:
Won't Fix
Status in OpenStack Compute (nova) xena series:
Won't Fix
Status in OpenStack Compute (nova) yoga series:
New
Status in nova package in Ubuntu:
Fix Released
Status in nova source package in Bionic:
Won't Fix
Status in nova source package in Focal:
In Progress
Status in nova source package in Jammy:
In Progress
Bug description:
******** SRU TEMPLATE AT THE BOTTOM *******
Hello,
This is very similar to pad.lv/1852437 (and the related blueprint at
https://blueprints.launchpad.net/nova/+spec/allow-disabling-cpu-
flags), but there is a very different and important nuance.
A customer I'm working with has two classes of blades that they're
trying to use. Their existing ones are Cascade Lake-based; they are
presently using the Cascadelake-Server-noTSX CPU model via
libvirt.cpu_model in nova.conf. Their new blades are Ice Lake-based,
which is a newer processor, which typically would also be able to run
based on the Cascade Lake feature set - except that these Ice Lake
processors lack the MPX feature defined in the Cascadelake-Server-
noTSX model.
The result of this is evident when I try to start nova on the new
blades with the Ice Lake CPUs. Even if I specify the following in my
nova.conf:
[libvirt]
cpu_mode = custom
cpu_model = Cascadelake-Server-noTSX
cpu_model_extra_flags = -mpx
That is not enough to allow Nova to start; it fails in the libvirt
driver in the _check_cpu_compatibility function:
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service Traceback (most recent call last):
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service File "/usr/lib/python3/dist-packages/nova/virt/libvirt/driver.py", line 771, in _check_cpu_compatibility
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service self._compare_cpu(cpu, self._get_cpu_info(), None)
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service File "/usr/lib/python3/dist-packages/nova/virt/libvirt/driver.py", line 8817, in _compare_cpu
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service raise exception.InvalidCPUInfo(reason=m % {'ret': ret, 'u': u})
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service nova.exception.InvalidCPUInfo: Unacceptable CPU info: CPU doesn't have compatibility.
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service 0
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service Refer to http://libvirt.org/html/libvirt-libvirt-host.html#virCPUCompareResult
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service During handling of the above exception, another exception occurred:
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service Traceback (most recent call last):
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service File "/usr/lib/python3/dist-packages/oslo_service/service.py", line 810, in run_service
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service service.start()
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service File "/usr/lib/python3/dist-packages/nova/service.py", line 173, in start
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service self.manager.init_host()
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service File "/usr/lib/python3/dist-packages/nova/compute/manager.py", line 1404, in init_host
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service self.driver.init_host(host=self.host)
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service File "/usr/lib/python3/dist-packages/nova/virt/libvirt/driver.py", line 743, in init_host
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service self._check_cpu_compatibility()
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service File "/usr/lib/python3/dist-packages/nova/virt/libvirt/driver.py", line 777, in _check_cpu_compatibility
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service raise exception.InvalidCPUInfo(msg)
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service nova.exception.InvalidCPUInfo: Configured CPU model: Cascadelake-Server-noTSX is not compatible with host CPU. Please correct your config and try again. Unacceptable CPU info: CPU doesn't have compatibility.
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service 0
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service Refer to http://libvirt.org/html/libvirt-libvirt-host.html#virCPUCompareResult
2022-12-15 17:20:59.562 1836708 ERROR oslo_service.service
If I make a custom libvirt CPU map file which removes the "<feature
name='mpx'/>" feature and specify that as the cpu_model instead, I am
able to make Nova start - so it does indeed seem to specifically be
that single feature which is blocking me. However, editing the
libvirt CPU mapping files is probably not the right way to fix this -
hence why I'm filing this bug, for discussion of how to support cases
like this.
Currently the only "proper" way I'm aware of to work around this right
now is to fall back to a Broadwell-based configuration which lacks the
"mpx" feature to use as a common baseline, but that's a much older
configuration than Cascade Lake and would mean missing out on all the
other features which are common in both Cascade Lake and Ice Lake. I
would rather if there were a way to use the Cascade Lake settings but
simply remove that "mpx" feature from use.
----
Steps to reproduce
==================
On an Ice Lake system lacking the MPX feature (e.g. /proc/cpuinfo
reporting model of "Intel(R) Xeon(R) Gold 5318Y"), specify the
following settings in nova.conf in libvirt settings:
[libvirt]
cpu_mode = custom
cpu_model = Cascadelake-Server-noTSX
cpu_model_extra_flags = -mpx
Then try to start nova.
Expected result
===============
Nova should start since Cascadelake-Server-noTSX is a subset of
Icelake-Server-noTSX, thus allowing the use of Cascadelake-Server-
noTSX as a common baseline model for both Cascade Lake and Ice Lake
servers.
Actual result
=============
Nova refuses to start, claiming the specified CPU model is
incompatible. The "cpu_model_extra_flags = -mpx" config option does
not help.
Environment
===========
Nova/OpenStack version: OpenStack Ussuri running on Ubuntu Focal.
Specifically, nova packages are at version 2:21.2.4-0ubuntu2.
Hypervisor: libvirt + KVM
Other relevant notes
====================
There are some other open related bugs. The removal of the MPX
feature in some Ice Lake processors has manifested in other ways as
well. These bugs are primarily in regards to the missing MPX feature
breaking how Ice Lake processors are detected, so the nuance is
somewhat different - however, they may be worth reviewing as well.
* https://gitlab.com/libvirt/libvirt/-/issues/304: bug regarding the
Icelake CPU maps in libvirt not working to detect certain Ice Lakes,
instead detecting them as Broadwell-noTSX-IBRS according to "virsh
capabilities" due to lacking the MPX feature. (I've personally tested
that removing the mpx feature from the associated CPU mapping files
allows for detecting as Ice Lake, but that's not the correct way to
fix this.)
There is also an interesting comment on this bug at
https://gitlab.com/libvirt/libvirt/-/issues/304#note_1065798706. It
basically implies that rather than looking at "virsh capabilities",
"virsh domcapabilities" should be used instead as it seems to more
correctly identify the CPU model even if there are disabled flags like
MPX.
* https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1978064:
Launchpad-side bug regarding the above issue as encountered in Ubuntu.
===============
SRU Description
===============
[Impact]
When using IceLake CPUs alongside CascadeLake CPUs, the Nova code does
not start due to comparing CPU models. It fails before even comparing
the flags. Unfortunately, IceLake CPUs are detected as having
compatibility with Broadwell, not CascadeLake. Using Broadwell as a
common denominator disables many modern features. The Libvirt upstream
team will not add specific support to IceLake [1]. The fix [2] in Nova
is to ignore CPU check (as a configurable workaround) as let libvirt
handle the added/removed flags, which is assumed to work for this
specific case.
[Test case]
Due to not having Icelake and Cascadelake CPUs in our usual lab for
testing of this specific scenario, the test case for this could be
either:
1) run for this SRU is running the charmed-openstack-tester [1]
against the environment containing the upgraded package (essentially
as it would be in a point release SRU) and expect the test to pass.
Test run evidence will be attached to LP.
2) manually deploy nova and the necessary openstack services to get
Nova to the code point of validating the issue in a single node. I
already achieved this and was able to test the fix by hacking the node
code to bypass the need of other services (conductor, keystone, mysql,
etc) but for a proper validation a clean installation (without any
hackery) is considered mandatory. In such case, the test case would
be:
a) Deploy nova and required services in an IceLake machine
b) Make sure the nova.conf has:
cpu_mode = custom
cpu_models = Cascadelake-Server-noTSX
cpu_model_extra_flags = -mpx
c) Check /var/log/nova/nova-compute.log for a successful nova-compute
service boot. It will not start properly without the fix, therefore
presenting the error:
2024-07-08 15:08:48.378 8399 CRITICAL nova [-] Unhandled error:
nova.exception.InvalidCPUInfo: Configured CPU model: Cascadelake-
Server-noTSX is not compatible with host CPU. Please correct your
config and try again. Unacceptable CPU info: CPU doesn't have
compatibility.
d) Install package containing the fix and confirm the successful nova-
compute service restart, not containing the error and containing this
instead:
2024-07-09 19:41:31.806 243487 DEBUG nova.virt.libvirt.driver [-] cpu compare xml: <cpu match="exact">
<model>Cascadelake-Server-noTSX</model>
<feature name="mpx" policy="disable"/>
</cpu>
[Regression Potential]
There is 1 new behavior introduced and 1 changed. The behavior
introduced is gated by a new config option that needs to be enabled,
and when enabled, it skips running the code. The behavior changed is
the one assumed by the default disabled value of the config option.
The fact that the code being backported in Yoga-Ussuri is exactly the
same as in currently Master (Caracal+), it means that no issues have
been found with the code across 4 releases, giving some confidence
that the code changed is unlikely to cause issues.
[Other Info]
[1] https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1978064
[2] https://review.opendev.org/c/openstack/nova/+/871969
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