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If the cpuset cgroup controller is disabled in /etc/libvirt/qemu.conf QEMU virtual machines can in principle use all host CPUs, even if they are hot plugged, if they have no explicit CPU affinity defined. However, there's libvirt code supposed to handle the situation where the libvirt daemon itself is not using all host CPUs. The code in qemuProcessInitCpuAffinity attempts to set an affinity mask including all defined host CPUs. Unfortunately, the resulting affinity mask for the process will not contain the offline CPUs. See also the sched_setaffinity(2) man page. That means that even if the host CPUs come online again, they won't be used by the QEMU process anymore. The same is true for newly hot plugged CPUs. So we are effectively preventing that QEMU uses all processors instead of enabling it to use them. It only makes sense to set the QEMU process affinity if we're able to actually grow the set of usable CPUs, i.e. if the process affinity is a subset of the online host CPUs. There's still the chance that for some reason the deliberately chosen libvirtd affinity matches the online host CPU mask by accident. In this case the behavior remains as it was before (CPUs offline while setting the affinity will not be used if they show up later on). Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com> Tested-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>