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SubjectRe: [PATCH 6/5] KVM: x86: fix periodic lapic timer with hrtimers
2016-10-25 07:39+0800, Wanpeng Li:
> 2016-10-24 23:27 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
>> 2016-10-24 17:09+0200, Paolo Bonzini:
>>> On 24/10/2016 17:03, Radim Krčmář wrote:
>> [...]
>>>
>>> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
>>>
>>> Go ahead, squash it into 5/5 and commit to kvm/queue. :)
>>
>> Did that, thanks.
>>
>> Wanpeng, the code is now under your name so please check it and/or
>> complain.
>
> This patch 6/5 incurred regressions.
>
> - The latency of the periodic mode which is emulated by VMX preemption
> is almost the same as periodic mode which is emulated by hrtimer.

Hm, what numbers are you getting?

When I ran the test with the original series, then it actually had worse
results with the VMX preemption than it did with the hrimer:

hlt average latency = 1464151
pause average latency = 1467605

htl tests the hrtimer, pause tests the VMX preemption. I just replaced
"hlt" with "pause" in the assembly loop.

The worse result was because the VMX preemption period was computed
incorrectly -- it was being added to now(). Some time passes between
the expiration and reading of now(), so this time was extending the
period while it shouldn't have.

If I run the test with [6/5], it gets sane numbers:

hlt average latency = 1465107
pause average latency = 1465093

The numbers are sane bacause the test is not computing latency (= how
long after the timer should have fired have we received the interrupt)
-- it is computing the duration of the period in cycles, which is much
better right now.

> - The oneshot mode test of kvm-unit-tests/apic_timer_latency.flat almost fail.

Oops, silly mistake -- apic_timer_expired() was in the 'else' branch in
[5/5] and I didn't invert the condition after moving it.

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 6244988418be..d7e74c8ec8ca 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1354,8 +1354,8 @@ static void start_sw_period(struct kvm_lapic *apic)
return;

if (apic_lvtt_oneshot(apic) &&
- ktime_after(apic->lapic_timer.target_expiration,
- apic->lapic_timer.timer.base->get_time())) {
+ !ktime_after(apic->lapic_timer.target_expiration,
+ apic->lapic_timer.timer.base->get_time())) {
apic_timer_expired(apic);
return;
}
Paolo, can you squash that?

> Btw, hope you can also apply the testcase for kvm-unit-tests. :)

I will have some comments, because it would be nicer if it measured the
latency ... expected_expiration is not computed correctly.

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