linux-kernel - Re: [RFC][PATCH 5/5] perfcounter: Add support for kernel hardware breakpoints

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Message-ID: <20090729003607.GC5104@nowhere>
Date:	Wed, 29 Jul 2009 02:36:09 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	prasad@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Anton Blanchard <anton@...ba.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Zhaolei <zhaolei@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 5/5] perfcounter: Add support for kernel hardware
	breakpoints

On Tue, Jul 28, 2009 at 09:24:37AM +0200, Peter Zijlstra wrote:
> On Tue, 2009-07-28 at 03:03 +0200, Frederic Weisbecker wrote:
> > > Thirdly, we can multiplex perf counters beyond their hardware maximum,
> > > something you simply cannot do for a debug interface.
> > 
> > 
> > Again, I'm stuck in what you mean by multiplexing here :-)
> 
> If you'd create say 16 breakpoint counters, we'd RR them over the 4
> available hardware breakpoints (or less when others are taken by someone
> else).
> 
> Since its all statistics anyway, we can simply scale the event counts up
> by the time-share they received.
>

Aah, ok I understand now.
But I fear it may kill the accuracy of the breakpoints statistics.
It's fine for a theoretical linear rate of breakpoint events.
But what happens if we are profiling something much more unstable
with a rain of hits in a small window of memory between large timeframes?
If we have a breakpoint inside this window of memory and this breakpoint
is not plugged, waiting for its turn in the RR, we loose this rain of events.

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