linux-kernel - Re: I.5 - Mmaped count

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Date:	Tue, 23 Jun 2009 10:33:06 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	eranian@...il.com
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Robert Richter <robert.richter@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andi Kleen <andi@...stfloor.org>,
	Maynard Johnson <mpjohn@...ibm.com>,
	Carl Love <cel@...ibm.com>,
	Corey J Ashford <cjashfor@...ibm.com>,
	Philip Mucci <mucci@...s.utk.edu>,
	Dan Terpstra <terpstra@...s.utk.edu>,
	perfmon2-devel <perfmon2-devel@...ts.sourceforge.net>
Subject: Re: I.5 - Mmaped count

stephane eranian writes:

> Unless you tell me that pc->index is marked invalid (0) when the
> event is not scheduled. I don't see how you can avoid reading
> the wrong value. I am assuming that is the event is not scheduled
> lock remains constant.

That's what happens; when pc->index == 0, the event is not scheduled
and the current count is in pc->offset.

> Assuming the event is active when you enter the loop and you
> read a value. How to get the timing information to scale the
> count?

At present you'd have to do a read().  It wouldn't be hard to add
fields to mmapped page to enable the user program to compute
up-to-date timing information from the TSC (x86) or timebase (powerpc)
value.  We'll do that.

Paul.
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