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

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Message-ID: <20130510102245.GA31235@dyad.programming.kicks-ass.net>
Date:	Fri, 10 May 2013 12:22:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.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

On Fri, May 10, 2013 at 12:18:23PM +0200, Ingo Molnar wrote:
> 
> * 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.

The part I have trouble with is that its a vague request and you'll get a vague
answer.

Its very similar to: 'do something' and getting a string of random bytes back.
Yes it is 'something', request fulfilled.

By doing the probing, userspace knows exactly what it asked and what it'll get.
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