** Description changed:
******** 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 the lab for testing of
- this specific scenario, the test case for this will be 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.
+ 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. The
- behavior changed is the one assumed by the default disabled value of the
- config option, and is not (in theory) intended to be the code path that
- addresses the bug. If we had the capability of testing the bug and fix
- in the lab, we could minimize risk by just introducing the config option
- and no further changes. On the other hand, the fact that the code being
- backported in Yoga is exactly the same as in currently Master
- (Caracal+), it means that no issues have been found with the code across
- 4 releases.
+ 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
** Changed in: nova/victoria
Status: New => Won't Fix
** Changed in: nova/wallaby
Status: New => Won't Fix
** Changed in: nova/xena
Status: New => Won't Fix
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https://bugs.launchpad.net/bugs/1999814
Title:
[SRU] Allow for specifying common baseline CPU model with disabled
feature
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