On Wed, 26 Jun 2019, Peter Zijlstra wrote: > On Wed, Jun 26, 2019 at 07:27:35PM +0000, Raslan, KarimAllah wrote: > > On Wed, 2019-06-26 at 21:21 +0200, Peter Zijlstra wrote: > > > On Wed, Jun 26, 2019 at 06:55:36PM +0000, Raslan, KarimAllah wrote: > > > > > > > > > > > If the host is completely in no_full_hz mode and the pCPU is dedicated to a > > > > single vCPU/task (and the guest is 100% CPU bound and never exits), you would > > > > still be ticking in the host once every second for housekeeping, right? Would > > > > not updating the mwait-time once a second be enough here? > > > > > > People are trying very hard to get rid of that remnant tick. Lets not > > > add dependencies to it. > > > > > > IMO this is a really stupid issue, 100% time is correct if the guest > > > does idle in pinned vcpu mode. > > > > One use case for proper accounting (obviously for a slightly relaxed definition > > or *proper*) is *external* monitoring of CPU utilization for scaling group > > (i.e. more VMs will be launched when you reach a certain CPU utilization). > > These external monitoring tools needs to account CPU utilization properly. > > That's utter nonsense; what's the point of exposing mwait to guests if > you're not doing vcpu pinning. For overloaded guests mwait makes no > sense what so ever. I think you misunderstood. The guests are pinned. What they can do today is monitor the guests utilization time through mwait/vmexit. If that goes over a certain threshold they can automatically launch more VMs to spread the load. With MWAIT in the guest this is gone... Thanks, tglx
