Re: [PATCH] KVM: selftests: Make hyperv_clock selftest more stable

[Maxim Levitsky]

On Wed, 2022-06-01 at 16:06 +0000, Sean Christopherson wrote:
> On Wed, Jun 01, 2022, Vitaly Kuznetsov wrote:
> > hyperv_clock doesn't always give a stable test result, especially
> > with
> > AMD CPUs. The test compares Hyper-V MSR clocksource (acquired
> > either
> > with rdmsr() from within the guest or KVM_GET_MSRS from the host)
> > against rdtsc(). To increase the accuracy, increase the measured
> > delay
> > (done with nop loop) by two orders of magnitude and take the mean
> > rdtsc()
> > value before and after rdmsr()/KVM_GET_MSRS.
> 
> Rather than "fixing" the test by reducing the impact of noise, can we
> first try
> to reduce the noise itself?  E.g. pin the test to a single CPU, redo
> the measurement

Pinning is a good idea overall, however IMHO should not be done in 
all KVM selftests, as vCPU migration itself can be source of bugs.


> if the test is interrupted (/proc/interrupts?), etc...  Bonus points
This is not feasable IMHO - timer interrupt alone can fire at rate of
1000 interrupts/s. Just while reading /proc/interurpts you probably get
few of interrupts.


> if that can
> be implemented as a helper or pair of helpers so that other tests
> that want to
> measure latency/time don't need to reinvent the wheel.
> 

Best regards,
	Maxim Levitsky