On Fri, 2 Feb 2018 14:53:50 +0100 Viktor Mihajlovski <mihajlov@xxxxxxxxxxxxxxxxxx> wrote: > On 01.02.2018 21:26, Eduardo Habkost wrote: > > On Thu, Feb 01, 2018 at 09:15:15PM +0100, Radim Krčmář wrote: > >> 2018-02-01 12:54-0500, Luiz Capitulino: > >>> > >>> Libvirt needs to know when a vCPU is halted. To get this information, > >> > >> I don't see why upper level management should care about that, a single > >> bit about halted state that can be incorrect at the time it is processed > >> seems of very limited use. > > > > I don't see why, either. > > > > I'm CCing libvir-list and the people involved in the code that > > added halt state to libvirt domain statistics. > > > I'll try to explain the motivation for the "halted" state exposure and > why it ended int the libvirt domain stats. > > s390 CPUs can be present in a system (e.g. after being hotplugged) but > be offline (disabled) in which case they are not used by the operating > system. In Linux disabled CPUs show a value of '0' in > /sys/devices/system/cpu/cpu<n>/online. If that's all you want, have you considered using the guest agent? > Higher level management software (on top of libvirt) can take advantage > of knowing whether a guest CPU is online and thus used or not. > Specifically it might not make sense to plug more CPUs if the guest OS > isn't using the CPUs at all. OK, so what's the algorithm used by the higher level management software where this all fits together? Something like: 1. Hotplug vCPU 2. Poll "halted" state 3. If "halted" becomes true, hotplug more vCPUs 4. If "halted" never becomes true, don't hotplug more CPUs If that's the case, then I guess grepping for State in /proc/qemu-pid/threadid/status will have the same end result, no? > > A disabled guest CPU is represented as halted in the QEMU object model > and can therefore be identified by the QMP query-cpus command. > > The initial patch proposal to expose this via virsh vcpuinfo was not > considered to be desirable because there was a concern that legacy > management software might be confused seeing halted vcpus. Therefore the > state information was added to the cpu domain statistics. > > One issue we're facing is that the semantics of "halted" are different > between s390 and at least x86. The question might be whether they are > different enough to grant a specific "disabled" indicator. > > [...] >
