On Wed, 26 Jun 2019, 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. Correct. We are going to see the same issue with UMWAIT/UMONITOR. If the timeout is set long enough by the admin, then a task can stay in user mode UMWAIT for a very long time. And we're going to account that as user time. That's not any different with a guest. You might go there and establish a shared page with the guest where the guest drops his internal accounting information. For trusted guests that might be a good approximation. For untrusted ones not so much, but then you just have to say, you occupy the CPU 100% in guest mode. If you idle there, none of my problems. Thanks, tglx
