linux-kernel - Re: IV.2 - Features impacting all counters

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Message-ID: <20090622115928.GQ24366@elte.hu>
Date:	Mon, 22 Jun 2009 13:59:28 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	eranian@...il.com
Cc:	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>,
	Paul Mackerras <paulus@...ba.org>,
	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: IV.2 - Features impacting all counters

> 2/ Features impacting all counters
>
> On some PMU models, e.g., Itanium, they are certain features which
> have an influence on all counters that are active. For instance,
> there is a way to restrict monitoring to a range of continuous
> code or data addresses using both some PMU registers and the debug
> registers.
>
> Given that the API exposes events (counters) as independent of
> each other, I wonder how range restriction could be implemented.

A solution is to make it a per-counter attribute and fail to 
schedule multiple counters at the same time when these constraints 
differ.

> Similarly, on Itanium, there are global behaviors. For instance,
> on counter overflow the entire PMU freezes all at once. That seems
> to be contradictory with the design of the API which creates the
> illusion of independence.
>
> What solutions do you propose?

We propose the same solution as last time: live with the small
imprecisions caused by such hardware limitations. We submit that
natural workload noise is probably far bigger than any such effect.
We could certainly list it as a platform limitation.
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