Bernhard Bock

OpenStack - Check for double CPU pins

In the “Mitaka” release of OpenStack, we had an issue that when using CPU pinning in instances, the scheduler did not take into account that some CPU cores were already occupied by other, also-pinned, instances. In order to find and fix this problem, a Python script can connect to the libvirt API and query pinned CPUs.

The script is run on all compute nodes via Ansible and exits with exit code 1 if a host has a double CPU pin. Therefore, all machines with a “failed” script need manual intervention.

#!/usr/bin/env python2

# Script to check if pinned CPUs are used more than once

import sys
import libvirt

cpus_used = [None] * 100
conn = libvirt.openReadOnly(None)

for domain_id in conn.listDomainsID():
    domain = conn.lookupByID(domain_id)
    pins = domain.vcpuPinInfo()
    for cpu in pins:
        # cpu is an bitfield (list of bools) with the length of available CPU cores
        # If a CPU is pinned, exactly one value is True, its index is indicating which
        # CPU we are talking about
        if cpu.count(True) == 1:

            # this list comprehension converts it to an int
            cpu_id = [i for i, x in enumerate(cpu) if x][0]

    	    if not cpus_used[cpu_id]:
                # haven't seen it used before. mark it as used
                cpus_used[cpu_id] = True
                # CPU was already used. print a message and exit with error
                print("CPU used twice: %d on VM %s" % (cpu_id,

        # no need to check further, manual intervention required

— Feb 10, 2019