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Date: Tue, 23 Sep 2008 22:03:45 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Martin Bligh <mbligh@google.com>
cc: Masami Hiramatsu <mhiramat@redhat.com>,
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Subject: Re: Unified tracing buffer
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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