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

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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