linux-kernel - Re: [PATCH 6/5] KVM: x86: fix periodic lapic timer with hrtimers

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Date:   Tue, 25 Oct 2016 13:43:35 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>,
        Yunhong Jiang <yunhong.jiang@...el.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [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@...hat.com>:
>> 2016-10-24 17:09+0200, Paolo Bonzini:
>>> On 24/10/2016 17:03, Radim Krčmář wrote:
>> [...]
>>>
>>> Reviewed-by: Paolo Bonzini <pbonzini@...hat.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.