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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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