[libvirt] libvirt to report cpuWait of a guest.

[Avi Weit]

Hi,

We are working on fixing CPU utilization calculation of guests (VMs) in 
Ceilometer metering service of OpenStack.

Per that fix, we retrieve, at the host level, se.statistics.wait_sum (or 
se.wait_sum in old versions) Linux kernel schedstat counter of the 
KVM/QEMU threads ? that represent the VM. That counter denotes the time in 
milliseconds the thread 'wants' to run (but not actually running because 
of a contention).

However, as Daniel Berrange mentioned in the fix review, it would be 
better to retrieve that information from libvirt instead and he asked me 
to raise this in this mailing list.

I could not notice that libvirt exposes such a counter.

It would be very good if virDomainInfo (
https://libvirt.org/html/libvirt-libvirt-domain.html#virDomainInfo) can be 
added with cpuWait in addition to cpuTime it currently has so that cpuWait 
will denote the total *time* the VM *wants* to run.

Adding cpuWait to any other location of domain is fine as well.

Thanks,
Avi Weit

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