linux-kernel - Re: [perfmon] Re: [perfmon2] perfmon2 merge news

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Message-ID: <20071114161325.GL6557@frankl.hpl.hp.com>
Date:	Wed, 14 Nov 2007 08:13:25 -0800
From:	Stephane Eranian <eranian@....hp.com>
To:	William Cohen <wcohen@...hat.com>
Cc:	Andi Kleen <andi@...stfloor.org>, akpm@...l.org,
	Robert Richter <robert.richter@....com>, gregkh@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [perfmon] Re: [perfmon2] perfmon2 merge news


On Wed, Nov 14, 2007 at 10:44:20AM -0500, William Cohen wrote:
> Andi Kleen wrote:
> 
> >>One approach does not prevent the other. Assuming you allow cr4.pce, then 
> >>nothing prevents
> >>a self-monitoring thread from reading the counters directly. You'll just 
> >>get the
> >>lower 32-bit of it. So if you read frequently enough, you should not have 
> >>a problem.
> >
> >Hmm? RDPMC is 64bit.
> 
> There are a number of processors that have 32-bit counters such as the IBM 
> power processors. On many x86 processors the upper bits of the counter are 
> sign extended from the lower 32 bits. Thus, one can only assume the lower 
> 32-bit are available. Roll over of values is quite possible (<2 seconds of 
> cycle count), so additional work needs to be done to obtain a valid value.
> 

Exactly, on Intel's only the bottom 32-bit actually are useable, the rest is
sign-extension. That's why it is okay for measuring small sections of code,
but that's it. On AMD, I think it is better. On Itanium you get the 47-bit worth.
Don't know about Power or Cell.

-- 
-Stephane
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