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

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Date:	Fri, 16 Nov 2012 16:56:27 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@...achi.com>,
	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>,
	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: [RFC PATCH 0/2] kvm/vmx: Output TSC offset

On Fri, Nov 16, 2012 at 10:05:54AM -0500, 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.
> 
> The code is pretty much ready, but I want to test it more before the 3.8
> window opens, which is why I'm waiting for 3.9.
> 
> You can see the latest version here:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
> branch: rfc/multi-buffers-v7
> 
> -- Steve

You need the tsc offset change event and the main events to be ordered,
so i can't see how this helps.

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