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[OpenStack Infra <1847367@xxxxxxxxxxxxxxxxxx>]

Reviewed:  https://review.opendev.org/695118
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=0e6aac3c2d97c999451da50537df6a0cbddeb4a6
Submitter: Zuul
Branch:    master

commit 0e6aac3c2d97c999451da50537df6a0cbddeb4a6
Author: Sean Mooney <work@xxxxxxxxxxxxxxx>
Date:   Wed Nov 20 00:13:03 2019 +0000

    add [libvirt]/max_queues config option
    
    This change adds a max_queues config option to allow
    operators to set the maximium number of virtio queue
    pairs that can be allocated to a virtio network
    interface.
    
    Change-Id: I9abe783a9a9443c799e7c74a57cc30835f679a01
    Closes-Bug: #1847367


** Changed in: nova
       Status: In Progress => Fix Released

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https://bugs.launchpad.net/bugs/1847367

Title:
  Images with hw:vif_multiqueue_enabled can be limited to 8 queues even
  if more are supported

Status in OpenStack Compute (nova):
  Fix Released

Bug description:
  Nova version: 18.2.3
  Release: Rocky
  Compute node OS: CentOS 7.3
  Compute node kernel: 3.10.0-327.13.1.el7.x86_64

  In https://bugs.launchpad.net/nova/+bug/1570631 and commit
  https://review.opendev.org/#/c/332660/, a bug was fixed by making the
  assumption that the kernel version should also dictate the max number
  of queues on the tap interface when setting
  hw:vif_multiqueue_enabled=True. It was decided that 3.x kernels have a
  max queue count of 8. Unfortunately not all distributions follow this,
  and CentOS/RHEL has supported up to 256 queues since at least 7.2 even
  with a 3.x kernel.

  The result of this is that a 20 core VM created in Mitaka will have 20
  queues enabled (because the limit of 8 had not been added). The very
  same host after being upgraded to Rocky will instead only give 8
  queues to the VM even though the kernel supports 256.

  Could a workaround option be implemented to disable this check, or
  manually define the max queue count?

  Snippet of drivers/net/tun.c from CentOS 7.2 kernel source code:
  /* MAX_TAP_QUEUES 256 is chosen to allow rx/tx queues to be equal
   * to max number of VCPUs in guest. */
  #define MAX_TAP_QUEUES 256
  #define MAX_TAP_FLOWS  4096

  Snippet from the 3.10.0 kernel code from https://elixir.bootlin.com/linux/v3.10/source/drivers/net/tun.c:
  /* DEFAULT_MAX_NUM_RSS_QUEUES were choosed to let the rx/tx queues allocated for
   * the netdevice to be fit in one page. So we can make sure the success of
   * memory allocation. TODO: increase the limit. */
  #define MAX_TAP_QUEUES DEFAULT_MAX_NUM_RSS_QUEUES
  #define MAX_TAP_FLOWS  4096

  In the above example, DEFAULT_MAX_NUM_RSS_QUEUES is set to 8.

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