linux-kernel - Re: [patch 0/3] [Announcement] Performance Counters for Linux

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Message-ID: <18744.61830.572384.96322@cargo.ozlabs.ibm.com>
Date:	Fri, 5 Dec 2008 20:16:54 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephane Eranian <eranian@...glemail.com>,
	Eric Dumazet <dada1@...mosbay.com>,
	Robert Richter <robert.richter@....com>,
	Arjan van de Veen <arjan@...radead.org>,
	Peter Anvin <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>
Subject: Re: [patch 0/3] [Announcement] Performance Counters for Linux

Peter Zijlstra writes:

> On Fri, 2008-12-05 at 18:57 +1100, Paul Mackerras wrote:
> > Peter Zijlstra writes:
> > 
> > > So, while most people would not consider two consecutive read() ops to
> > > be close or near the same time, due to preemption and such, that is
> > > taken away by the fact that the counters are task local time based - so
> > > preemption doesn't affect thing. Right?
> > 
> > I'm sorry, I don't follow the argument here.  What do you mean by
> > "task local time based"?
> 
> time only flows when the task is running.

Right - but the monitored task is running while the monitoring task is
running.  So time is flowing for the monitored task between the two
reads done by the monitoring task, meaning that you can't actually
relate the two values you read with any precision.

Paul.
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