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

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Date:	Mon, 20 Oct 2008 13:10:02 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	"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 Fri, 17 Oct 2008, Mathieu Desnoyers wrote:
> 
> Hrm, on such systems
> - *large* amount of cpus
> - no synchronized TSCs
> 
> What would be the best approach to order events ?

My strong opinion has been - for a longish while now, and independently of 
any timestamping code - that we should be seriously looking at basically 
doing essentially a "ntp" inside the kernel to give up the whole idiotic 
notion of "synchronized TSCs". Yes, TSC's are often synchronized, but even 
when they are, we might as well _think_ of them as not being so.

In other words, instead of expecting internal clocks to be synchronized, 
just make the clock be a clock network of independent TSC domains. The 
domains could in theory be per-package (assuming TSC is synchronized at 
that level), but even if we _could_ do that, we'd probably still be better 
off by simply always doing it per-core. If only because then the reading 
would be per-core.

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.

		Linus
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