linux-kernel - Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset

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Date:	Tue, 20 Nov 2012 19:38:53 +0900
From:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	David Sharp <dhsharp@...gle.com>, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Joerg Roedel <joerg.roedel@....com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>,
	yrl.pp-manager.tt@...achi.com,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: Re: Re: [RFC PATCH 0/2] kvm/vmx: Output TSC offset

Hi Steven,

Sorry for the late reply.

(2012/11/17 0:05), Steven Rostedt wrote:
> On Wed, 2012-11-14 at 17:26 +0900, Yoshihiro YUNOMAE wrote:
>> Thank you for commenting on my patch set.
>>
>> (2012/11/14 11:31), Steven Rostedt wrote:
>>> On Tue, 2012-11-13 at 18:03 -0800, David Sharp wrote:
>>>> On Tue, Nov 13, 2012 at 6:00 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>>>> On Wed, 2012-11-14 at 10:36 +0900, Yoshihiro YUNOMAE wrote:
>>>>>
>>>>>> To merge the data like previous pattern, we apply this patch set. Then, we can
>>>>>> get TSC offset of the guest as follows:
>>>>>>
>>>>>> $ dmesg | grep kvm
>>>>>> [   57.717180] kvm: (2687) write TSC offset 18446743360465545001, now clock ##
>>>>>>                        ^^^^                   ^^^^^^^^^^^^^^^^^^^^            |
>>>>>>                        PID                         TSC offset                 |
>>>>>>                                                              HOST TSC value --+
>>>>>>
>>>>>
>>>>> Using printk to export something like this is IMO a nasty hack.
>>>>>
>>>>> Can't we create a /sys or /proc file to export the same thing?
>>>>
>>>> Since the value changes over the course of the trace, and seems to be
>>>> part of the context of the trace, I think I'd include it as a
>>>> tracepoint.
>>>>
>>>
>>> I'm fine with that too.
>>
>> Using some tracepoint is a nice idea, but there is one problem. Here,
>> our discussion point is "the event which TSC offset is changed does not
>> frequently occur, but the buffer must keep the event data."
>
> If you can hold off a bit, for the 3.9 window, I plan on pushing
> multiple buffers for ftrace. That is, you can create a separate buffer
> just for the TSC offset events:
>
> cd /sys/kernel/debug
> echo tsc > instances/new
> echo 1 > instances/tsc/events/tsc/offset/enable
>
> Then the buffer will be used only for that event.

That's good. The tracepoint will output as follows:

qemu-kvm-12345  [000] ....123456789: kvm_write_tsc_offset:
now_tsc=123456789 previous_offset=0 next_offset=123456780

Thanks,
-- 
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@...achi.com


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