Re: [patch 2/2] KVM: x86: add option to advance tscdeadline hrtimer expiration

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Dec 11, 2014 at 12:48:36PM -0800, Andy Lutomirski wrote:
> On 12/10/2014 07:07 PM, Marcelo Tosatti wrote:
> > On Thu, Dec 11, 2014 at 12:37:57AM +0100, Paolo Bonzini wrote:
> >>
> >>
> >> On 10/12/2014 21:57, Marcelo Tosatti wrote:
> >>> For the hrtimer which emulates the tscdeadline timer in the guest,
> >>> add an option to advance expiration, and busy spin on VM-entry waiting
> >>> for the actual expiration time to elapse.
> >>>
> >>> This allows achieving low latencies in cyclictest (or any scenario 
> >>> which requires strict timing regarding timer expiration).
> >>>
> >>> Reduces cyclictest avg latency by 50%.
> >>>
> >>> Note: this option requires tuning to find the appropriate value 
> >>> for a particular hardware/guest combination. One method is to measure the 
> >>> average delay between apic_timer_fn and VM-entry. 
> >>> Another method is to start with 1000ns, and increase the value
> >>> in say 500ns increments until avg cyclictest numbers stop decreasing.
> >>
> >> What values are you using in practice for the parameter?
> > 
> > 7us.
> 
> 
> It takes 7us to get from TSC deadline expiration to the *start* of
> vmresume?  That seems rather extreme.
> 
> Is it possible that almost all of that latency is from deadline
> expiration to C-state exit?  If so, can we teach the timer code to wake
> up early to account for that?  We're supposed to know our idle exit
> latency these days.

7us includes: 

idle thread wakeup
idle schedout
ksoftirqd schedin
ksoftirqd schedout
qemu schedin
vm-entry

C-states are disabled of course.