linux-kernel - Re: PROBLEM: no cpu MHz in /proc/cpuinfo on 2.6.25.4-rt6

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Date:	Wed, 11 Jun 2008 15:39:24 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	markh@...pro.net
CC:	Arjan van de Ven <arjan@...radead.org>,
	"linux-os (Dick Johnson)" <linux-os@...logic.com>,
	Jan Engelhardt <jengelh@...ozas.de>,
	"Kok, Auke" <auke-jan.h.kok@...el.com>,
	"solsTiCe d'Hiver" <solstice.dhiver@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: no cpu MHz in /proc/cpuinfo on 2.6.25.4-rt6

Mark Hounschell wrote:

> If they are not the same how do you know at what rate the tsc is counting.

You don't, at least not easily.  The timekeeping subsystem knows, but it 
might be unstable, or change with cpu frequency or cpu going idle, etc.

> But back to "how usable is rdtsc for time", is there a
> 
> clock_gettime(CLOCK_MONOTONIC, &time) equivalent that can be used in the kernel?

With current kernels it looks like ktime_get_ts() should give exactly 
the same results, since it's how that userspace function is implemented. 
  With a stable tsc it seems like it should give full accuracy.

Chris
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