On Wed, Jun 26, 2019 at 06:16:08PM +0200, Peter Zijlstra wrote: > 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. Considering that the CPUs aren't doing anything and if you do say the IPIs "only" 100/second - that would be so small but give you a big benefit in properly accounting the guests. But perhaps there are other ways too to "snoop" if a guest is sitting on an MWAIT?
