Date: Fri, 10 May 2013 12:31:12 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Jiri Olsa <jolsa@redhat.com>, linux-kernel@vger.kernel.org,
        Corey Ashford <cjashfor@linux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Ingo Molnar <mingo@elte.hu>,
        Namhyung Kim <namhyung@kernel.org>, Paul Mackerras <paulus@samba.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Andi Kleen <ak@linux.intel.com>, David Ahern <dsahern@gmail.com>,
        Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH 0/9] perf: Adding better precise_ip field handling
Message-ID: <20130510103112.GA18755@gmail.com>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, May 10, 2013 at 12:18:23PM +0200, Ingo Molnar wrote:
> > 
> > * Peter Zijlstra <peterz@infradead.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.

PERF_COUNT_HW_CPU_CYCLES_PRECISE is not a vague request at all: it means 
'get me the most precise cycles profiling available on this system'.

Thanks,

	Ingo
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