linux-kernel - Re: [RFC patch 15/15] LTTng timestamp x86

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Date:	Mon, 20 Oct 2008 14:38:37 -0700
From:	"john stultz" <johnstul@...ibm.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Mathieu Desnoyers" <mathieu.desnoyers@...ymtl.ca>,
	"Luck, Tony" <tony.luck@...el.com>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"David Miller" <davem@...emloft.net>,
	"Ingo Molnar" <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC patch 15/15] LTTng timestamp x86

On Mon, Oct 20, 2008 at 1:10 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> I think it's a mistake for us to maintain a single clock for
> gettimeofday() (well, "getnstimeofday" and the whole "clocksource_read()"
> crud to be technically correct). And sure, I bet clocksource_read() can do
> various per-CPU things and try to do that, but it's complex and pretty
> generic code, and as far as I know none of the clocksources have even
> tried. The TSC clocksource read certainly does not (it just does a very
> similar horrible "at least don't go backwards" crud that the LTTng patch
> suggested).
>
> So I think we should make "xtime" be a per-CPU thing, and add support for
> per-CPU clocksources. And screw that insane "mark_tsc_unstable()" thing.
>
> And if we did it well, we migth be able to get good timestamps that way
> too.

Personally I'd been hoping that the experiments in the trace
timestamping code would provide a safe area of experimentation before
we adapt it to the TSC clocksource implementation for
getnstimeofday(). Earlier I know Andi and Jiri were working on such a
per-cpu TSC clocksource, but I don't know where it ended up.

I'm not quite sure I followed your per-cpu xtime thoughts.  Could you
explain further your thinking as to why the entire timekeeping
subsystem should be per-cpu instead of just keeping that back in the
arch-specific clocksource implementation?  In other words, why keep
things synced at the nanosecond level instead of keeping the per-cpu
TSC synched at the cycle level?

thanks
-john
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