linux-kernel - Re: Unified tracing buffer

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Message-ID: <alpine.LFD.1.10.0809232140090.2038@apollo.tec.linutronix.de>
Date:	Tue, 23 Sep 2008 21:41:40 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Martin Bligh <mbligh@...gle.com>
cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>, darren@...art.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	systemtap-ml <systemtap@...rces.redhat.com>
Subject: Re: Unified tracing buffer

On Tue, 23 Sep 2008, Martin Bligh wrote:

> On Tue, Sep 23, 2008 at 12:36 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Tue, 23 Sep 2008, Masami Hiramatsu wrote:
> >> # cat /sys/devices/system/cpu/cpu0/cpuidle/state3/usage
> >> 171210
> >
> > C3 stops the TSC. So depending on how many C3 entries you have on the
> > different cores, your TSCs will drift apart. Some BIOSes do even a
> > lousy job trying to fixup the TSCs on exit from C3, which makes things
> > even worse.
> >
> >>     C1:                  type[C1] promotion[--] demotion[--] latency[001] usage[00000016] duration[00000000000000000000]
> >>     C2:                  type[C2] promotion[--] demotion[--] latency[001] usage[00037969] duration[00000000000024288003]
> >>     C3:                  type[C3] promotion[--] demotion[--] latency[057] usage[00171818] duration[00000000001881257636]
> >>
> >> Could these help you?
> >
> > Yup, explains your TSC observation. Nothing we can do about. Broken by
> > system design :( Welcome in the wonderful world of Inhell/BIOS/ACPI !
> 
> We have linux patches that sync the TSC on exit_idle. I'll see if I can get
> Michael to send them out.

Are you sure that they sync it precicely enough that there is no user
space observable way of time going backwards between cores ?

Thanks,

	tglx
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