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

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Date:	Sat, 18 Oct 2008 19:50:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
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>,
	"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>,
	"ltt-dev@...ts.casi.polymtl.ca" <ltt-dev@...ts.casi.polymtl.ca>,
	Michael Davidson <md@...gle.com>
Subject: Re: [RFC patch 15/15] LTTng timestamp x86


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> And if you make all these linear interpolations be per-CPU (so you 
> have per-CPU offsets and frequencies) you never _ever_ need to touch 
> any shared data at all, and you know you can scale basically 
> perfectly.
> 
> Your linear interpolations may not be _perfect_, but you'll be able to 
> get them pretty damn near. In fact, even if the TSC's aren't 
> synchronized at all, if they are at least _individually_ stable (just 
> running at slightly different frequencies because they are in 
> different clock domains, and/or at different start points), you can 
> basically perfect the precision over time.

there's been code submitted by Michael Davidson recently that looked 
interesting, which turns the TSC into such an entity:

    http://lkml.org/lkml/2008/9/25/451

The periodic synchronization uses the hpet, but it thus allows lockless 
and globally correct readouts of the TSC .

And that would match the long term goal as well: the hw should do this 
all automatically. So perhaps we should have a trace_clock() after all, 
independent of sched_clock(), and derived straight from RDTSC.

The approach as propoed has a couple of practical problems, but if we 
could be one RDTSC+multiplication away from a pretty good timestamp that 
would be rather useful, very fast and very robust ...

	Ingo
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