Public bug reported:
Upstream Linux kernel has removed[*] the 'perf cqm' (Cache
Quality-of-Service Monitoring) from the following kernels onwards:
[linux]$> git tag --contains c39a0e2
v4.14
Impact for OpenStack / Nova
---------------------------
Quoting the summary from Dan Berrangé from a downstream bug (with some
edits, references and formatting):
- Libvirt supports enabling perf event reporting per guest using <perf
../> XML in guest XML
https://libvirt.org/formatdomain.html#elementsPerf
- OpenStack has abiity to enable this support by using
/etc/nova/nova.conf setting "enabled_perf_events" in [libvirt]
section
- Although libvirt supports many events, OpenStack only supports the
'cmt', 'mbmt' and 'mbml' perf events
- Upstream Linux kernel decided the perf framework integration with
'cmt', 'mbmt' and 'mbml' events was broken by design and entirely
deleted it[*]
- Upstream kernel has provided a new approach to 'cmt', 'mbmt' and
'mbml' info reporting that is *not* using perf framework
- There's unlikely to be any way for libvirt to make this
functionality magically re-appear, given the kernel changes. The new
approach is completely incompatible with what was done before.
IOW, if someone has set "enabled_perf_events" in /etc/nova/nova.conf
previously, they will be unable to start any guest, once they upgrade to
any kernels that has backported the commit: c39a0e2 ("x86/perf/cqm: Wipe
out perf based cqm")[*].
[*]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c39a0e2
** Affects: nova
Importance: Undecided
Status: New
** Tags: libvirt
** Tags added: libvirt
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https://bugs.launchpad.net/bugs/1751073
Title:
[Regression] Nova's 'enabled_perf_events' feature will be broken with
Linux Kernel 4.14+
Status in OpenStack Compute (nova):
New
Bug description:
Upstream Linux kernel has removed[*] the 'perf cqm' (Cache
Quality-of-Service Monitoring) from the following kernels onwards:
[linux]$> git tag --contains c39a0e2
v4.14
Impact for OpenStack / Nova
---------------------------
Quoting the summary from Dan Berrangé from a downstream bug (with some
edits, references and formatting):
- Libvirt supports enabling perf event reporting per guest using <perf
../> XML in guest XML
https://libvirt.org/formatdomain.html#elementsPerf
- OpenStack has abiity to enable this support by using
/etc/nova/nova.conf setting "enabled_perf_events" in [libvirt]
section
- Although libvirt supports many events, OpenStack only supports the
'cmt', 'mbmt' and 'mbml' perf events
- Upstream Linux kernel decided the perf framework integration with
'cmt', 'mbmt' and 'mbml' events was broken by design and entirely
deleted it[*]
- Upstream kernel has provided a new approach to 'cmt', 'mbmt' and
'mbml' info reporting that is *not* using perf framework
- There's unlikely to be any way for libvirt to make this
functionality magically re-appear, given the kernel changes. The new
approach is completely incompatible with what was done before.
IOW, if someone has set "enabled_perf_events" in /etc/nova/nova.conf
previously, they will be unable to start any guest, once they upgrade to
any kernels that has backported the commit: c39a0e2 ("x86/perf/cqm: Wipe
out perf based cqm")[*].
[*]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c39a0e2
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