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[Thierry Carrez <thierry.carrez+lp@xxxxxxxxx>]

** Changed in: nova
       Status: Fix Committed => Fix Released

** Changed in: nova
    Milestone: None => liberty-1

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https://bugs.launchpad.net/bugs/1454451

Title:
  simultaneous boot of multiple instances leads to cpu pinning overlap

Status in OpenStack Compute (Nova):
  Fix Released

Bug description:
  I'm running into an issue with kilo-3 that I think is present in
  current trunk.  Basically it results in multiple instances (with
  dedicated cpus) being pinned to the same physical cpus.

  I think there is a race between the claimed CPUs of an instance being
  persisted to the DB, and the resource audit scanning the DB for
  instances and subtracting pinned CPUs from the list of available CPUs.

  The problem only shows up when the following sequence happens:
  1) instance A (with dedicated cpus) boots on a compute node
  2) resource audit runs on that compute node
  3) instance B (with dedicated cpus) boots on the same compute node

  So you need to either be booting many instances, or limiting the valid
  compute nodes (host aggregate or server groups), or have a small
  cluster in order to hit this.

  The nitty-gritty view looks like this:

  When booting up an instance we hold the COMPUTE_RESOURCE_SEMAPHORE in
  compute.resource_tracker.ResourceTracker.instance_claim() and that
  covers updating the resource usage on the compute node. But we don't
  persist the instance numa topology to the database until after
  instance_claim() returns, in
  compute.manager.ComputeManager._build_instance().  Note that this is
  done *after* we've given up the semaphore, so there is no longer any
  sort of ordering guarantee.

  compute.resource_tracker.ResourceTracker.update_available_resource()
  then aquires COMPUTE_RESOURCE_SEMAPHORE, queries the database for a
  list of instances and uses that to calculate a new view of what
  resources are available. If the numa topology of the most recent
  instance hasn't been persisted yet, then the new view of resources
  won't include any pCPUs pinned by that instance.

  compute.manager.ComputeManager._build_instance() runs for the next
  instance and based on the new view of available resources it allocates
  the same pCPU(s) used by the earlier instance. Boom, overlapping
  pinned pCPUs.

  
  Lastly, the same bug applies to the compute.manager.ComputeManager.rebuild_instance() case.  It uses the same pattern of doing the claim and then updating the instance numa topology after releasing the semaphore.

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