[Bug 1978489] Re: libvirt / cgroups v2: cannot boot instance with more than 16 CPUs

[James Page]

I think that the challenge of how to update the cpu tuning for all
existing running instances is solvable.

a) quota:cpu_* is an additional property for a flavor and as such can be
updated (applying to new instances created).

b) Using the virsh tool, its possible to live set the scheduling tuning
on a running instance - for example:

sudo virsh schedinfo instance-0008905c --config --live --set
cpu_shares=2048

That obviously needs tailoring for the actual running
environment/instances.

That does not however deal with the in-balance between instances created
before and post update with no flavor extra-specs defined.

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https://bugs.launchpad.net/bugs/1978489

Title:
  libvirt / cgroups v2: cannot boot instance with more than 16 CPUs

Status in OpenStack Compute (nova):
  In Progress
Status in nova package in Ubuntu:
  Confirmed
Status in nova source package in Jammy:
  Fix Committed

Bug description:
  Description
  ===========

  Using the libvirt driver and a host OS that uses cgroups v2 (RHEL 9,
  Ubuntu Jammy), an instance with more than 16 CPUs cannot be booted.

  Steps to reproduce
  ==================

  1. Boot an instance with 10 (or more) CPUs on RHEL 9 or Ubuntu Jammy
  using Nova with the libvirt driver.

  Expected result
  ===============

  Instance boots.

  Actual result
  =============

  Instance fails to boot with a 'Value specified in CPUWeight is out of
  range' error.

  Environment
  ===========

  Originially report as a libvirt but in RHEL 9 [1]

  Additional information
  ======================

  This is happening because Nova defaults to 1024 * (# of CPUs) for the
  value of domain/cputune/shares in the libvirt XML. This is then passed
  directly by libvirt to the cgroups API, but cgroups v2 has a maximum
  value of 10000. 10000 / 1024 ~= 9.76

  [1] https://bugzilla.redhat.com/show_bug.cgi?id=2035518

  
  ====================================

  Ubuntu SRU Details:

  [Impact]
  See above.

  [Test Case]
  See above.

  [Regression Potential]
  We've had this change in other jammy-based versions of the nova package for a while now, including zed, antelope, bobcat.

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