linux-kernel - Re: Unified tracing buffer

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Date:	Tue, 23 Sep 2008 22:03:45 +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:

> >> > 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 ?
> 
> I think the tolerance is about 500 cycles. If that's not sufficient, I guess
> we'll have to either live with some slight misordering (which people have
> pointed out is kind of inevitable anyway) on these broken machines?
> It was sufficient for what we were using it for, but maybe not for everyone.

Well, I dont care about the trace reordering at all. I care about user
space visible time going backwards issues observed via the
gettimeofday vsyscall. 500 cycles should be fine, I doubt that we can
migrate in less than that :)

I guess you try this only for machines where the TSC runs with
constant frequency, right ?

Looking forward to your patches.

Thanks,

	tglx


 
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