search for: kvm_sched_clock

> On Oct 4, 2018, at 1:11 AM, Peter Zijlstra <peterz at infradead.org> wrote: > >> On Thu, Oct 04, 2018 at 09:54:45AM +0200, Vitaly Kuznetsov wrote: >> I was hoping to hear this from you :-) If I am to suggest how we can >> move forward I'd propose: >> - Check if pure TSC can be used on SkyLake+ systems (where TSC scaling >> is supported). >> -

> On Oct 4, 2018, at 1:11 AM, Peter Zijlstra <peterz at infradead.org> wrote: > >> On Thu, Oct 04, 2018 at 09:54:45AM +0200, Vitaly Kuznetsov wrote: >> I was hoping to hear this from you :-) If I am to suggest how we can >> move forward I'd propose: >> - Check if pure TSC can be used on SkyLake+ systems (where TSC scaling >> is supported). >> -

...een CPUs and the > > code compensates for that. > > But if we?re in a KVM guest, then the clock will jump around on the > same *vCPU* when the vCPU migrates. Urgh yes.. > But I don?t see how kvmclock helps here, since I don?t think it?s used > for sched_clock. I get hits on kvm_sched_clock, but haven't looked further. Anyway, Most of the argument still holds, either TSC is synced or it is not and it _really_ should not be used. Not much middle ground there.

...or that. >> >> But if we?re in a KVM guest, then the clock will jump around on the >> same *vCPU* when the vCPU migrates. > > Urgh yes.. > >> But I don?t see how kvmclock helps here, since I don?t think it?s used >> for sched_clock. > > I get hits on kvm_sched_clock, but haven't looked further. Ok, so KVM is using the per-vCPU pvclock data for sched_clock. Which hopefully does something intelligent. Regardless of any TSC syncing issues, a paravirt clock should presumably be used for sched_clock to account for time that the vCPU was stopped. It would be...

...or that. >> >> But if we?re in a KVM guest, then the clock will jump around on the >> same *vCPU* when the vCPU migrates. > > Urgh yes.. > >> But I don?t see how kvmclock helps here, since I don?t think it?s used >> for sched_clock. > > I get hits on kvm_sched_clock, but haven't looked further. Ok, so KVM is using the per-vCPU pvclock data for sched_clock. Which hopefully does something intelligent. Regardless of any TSC syncing issues, a paravirt clock should presumably be used for sched_clock to account for time that the vCPU was stopped. It would be...