yahoo-eng-team team mailing list archive

[Kashyap Chamarthy <kchamart@xxxxxxxxxx>]

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

-- 
You received this bug notification because you are a member of Yahoo!
Engineering Team, which is subscribed to OpenStack Compute (nova).
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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