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Message-ID: <CANRm+CzFxyeVYMwq_+eZkCVc3ZdmC+kV2tdkyD1H_0XWO0yX9A@mail.gmail.com>
Date: Thu, 27 Oct 2016 10:33:51 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Radim Krčmář <rkrcmar@...hat.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
2016-10-26 22:01 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2016-10-26 14:08+0800, Wanpeng Li:
>> 2016-10-26 14:02 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
>>> 2016-10-25 19:43 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>>>> I will have some comments, because it would be nicer if it measured the
>>>> latency ... expected_expiration is not computed correctly.
>>>
>>> It measured the latency from guest programs the clock event device to
>>> interrupt injected to guest after timer fire.
>
> No. It never computed the time when the timer fires, the test measured
> the duration of the period.
>
> Imagine that the dashed line below is a timeline. Pipe is idealized
> firing of the periodic timer and caret is the time when the guest read
> time in the interrupt. The number below caret is the latency.
>
> The period is 7.
>
> --------------------------------------------
> | | | | | | |
> ^ ^ ^ ^ ^ ^ ^
> 1 2 3 1 2 1 1
>
> The test would report "latencies" as:
>
> 1 1 1 -2 1 -1 0
>
> because it used now() + period to compute the next expected expiration
>
> Similarly in this case,
> --------------------------------------------
> | | | | | | |
> ^ ^ ^ ^ ^ ^
> 6 6 6 6 6 6
>
> The latency is always 6, but the test would report
>
> 6 0 0 0 0 0
>
> And if we improved the latency by 1, you'd only see the difference in
> the first number. The test measured the duration of the period.
Agreed, thanks for the details. :)
Regards,
Wanpeng Li
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