On Wed, Jun 26, 2019 at 10:54:13AM -0400, Konrad Rzeszutek Wilk wrote: > On Wed, Jun 26, 2019 at 12:33:30PM +0200, Thomas Gleixner wrote: > > On Wed, 26 Jun 2019, Wanpeng Li wrote: > > > After exposing mwait/monitor into kvm guest, the guest can make > > > physical cpu enter deeper cstate through mwait instruction, however, > > > the top command on host still observe 100% cpu utilization since qemu > > > process is running even though guest who has the power management > > > capability executes mwait. Actually we can observe the physical cpu > > > has already enter deeper cstate by powertop on host. Could we take > > > cstate into consideration when accounting cputime etc? > > > > If MWAIT can be used inside the guest then the host cannot distinguish > > between execution and stuck in mwait. > > > > It'd need to poll the power monitoring MSRs on every occasion where the > > accounting happens. > > > > This completely falls apart when you have zero exit guest. (think > > NOHZ_FULL). Then you'd have to bring the guest out with an IPI to access > > the per CPU MSRs. > > > > I assume a lot of people will be happy about all that :) > > There were some ideas that Ankur (CC-ed) mentioned to me of using the perf > counters (in the host) to sample the guest and construct a better > accounting idea of what the guest does. That way the dashboard > from the host would not show 100% CPU utilization. But then you generate extra noise and vmexits on those cpus, just to get this accounting sorted, which sounds like a bad trade.
