linux-kernel - Re: [PATCH 2/2] tracing/function-graph-tracer: use the more lightweight local clock

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Message-ID: <20090305113828.GA9548@elte.hu>
Date:	Thu, 5 Mar 2009 12:38:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] tracing/function-graph-tracer: use the more
	lightweight local clock


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> > There will be one problem though: function trace elapsed 
> > time measurements across idle. Those are not correctly 
> > measured by the local clock.
> 
> Oh, why? If I'm not wrong it uses sched_clock() which uses tsc 
> on x86. And this register always goes forward at the same 
> rate. Unless some Cpu decrease their frequency while beeing 
> idle for some time and then become unreliable?

TSC stops in idle on many CPUs.

	Ingo
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