linux-kernel - Re: [PATCH 0/9] perf: Adding better precise

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Date:	Fri, 10 May 2013 12:18:23 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	David Ahern <dsahern@...il.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 0/9] perf: Adding better precise_ip field handling

* Peter Zijlstra <peterz@...radead.org> wrote:

> > so you mean just detect that by opening events with increasing precise 
> > and see how far we could get.. could be I guess, though the 'precise' 
> > sysfs attribute seems more fit to me
> 
> The other way around, start at ppp end at !p, then use the one that 
> worked.

Really, instead of this silly 'probing until it works' notion, how about 
the radical idea that we pass to the kernel our request, and the kernel 
fulfills our wish to the best of its ability?

This could be done as a new PERF_COUNT_HW_CPU_CYCLES_PRECISE event, to 
which tooling could migrate, without changing existing semantics.

The problem with the complex probing is that it's totally unnecessary 
complexity that results from lack of passing the right information to the 
kernel. Forcing that will only hinder user-space adoption of our precise 
profiling facilities.

Thanks,

	Ingo
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