Comment 22 for bug 1413540

This does not appear to be specific to OpenStack, nor tempest. I've reproduced with Trusty on Trusty on Trusty, vanilla qemu/kvm.

Simplified reproducer, with an existing MAAS cluster:

@L0 baremetal:
 - Create a Trusty bare metal host from daily images.
 - sudo apt-get update -y && sudo apt-get -y install uvtool
 - sudo uvt-simplestreams-libvirt sync release=trusty arch=amd64
 - sudo uvt-simplestreams-libvirt query
 - ssh-keygen
 - sudo uvt-kvm create --memory 2048 trusty-vm release=trusty
 - sudo virsh shutdown trusty-vm
 - # edit the /etc/libvirt/qemu/trusty-vm.xml to enable serial console dump to file:
    <serial type='file'>
      <source path='/tmp/trusty-vm-console.log'/>
      <target port='0'/>
    </serial>
    <console type='file'>
      <source path='/tmp/trusty-vm-console.log'/>
      <target type='serial' port='0'/>
    </console>
 - sudo virsh define /etc/libvirt/qemu/trusty-vm.xml
 - sudo virsh start trusty-vm
 - # confirm console output:
 - sudo tailf /tmp/trusty-vm-console.log
 - # take note of the VM's IP:
 - sudo uvt-kvm ip trusty-vm
 - # ssh into the new vm.

@L1 "trusty-vm":
 - sudo apt-get update -y && sudo apt-get -y install uvtool
 - sudo uvt-simplestreams-libvirt sync release=trusty arch=amd64
 - sudo uvt-simplestreams-libvirt query
 - ssh-keygen
 - # change .122. to .123. in /etc/libvirt/qemu/networks/default.xml
 - # make sure default.xml is static linked inside /etc/libvirt/qemu/networks
 - sudo reboot # for good measure
 - sudo uvt-kvm create --memory 768 trusty-nest release=trusty
 - # take note of the nested VM's IP
 - sudo uvt-kvm ip trusty-vm
 - # ssh into the new vm.

@L2 "trusty-nest":
 - sudo apt-get update && sudo apt-get install stress
 - stress -c 1 -i 1 -m 1 -d 1 -t 600

Now watch the "trusty-vm" console for: [ 496.076004] BUG: soft lockup - CPU#0 stuck for 23s! [ksmd:36]. It happens to me within a couple of minutes. Then, both L1 and L2 become unreachable indefinitely, with two cores on L0 stuck at 100%.