linux-kernel - Re: [PATCH] KVM: selftests: Make hyperv

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Message-ID: <87ee07nq8i.fsf@redhat.com>
Date:   Thu, 02 Jun 2022 15:34:37 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: selftests: Make hyperv_clock selftest more stable

Sean Christopherson <seanjc@...gle.com> writes:

> 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
> if the test is interrupted (/proc/interrupts?), etc...  Bonus points 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.

While I'm not certain task migration to another CPU was always the
problem here (maybe the measured interval is too short anyway), I agree
these are good ideas, I'll look into them, thanks!

-- 
Vitaly