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

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Message-ID: <20130510092741.GE3039@dyad.programming.kicks-ass.net>
Date:	Fri, 10 May 2013 11:27:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	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 Thu, May 09, 2013 at 05:20:22PM +0200, Jiri Olsa wrote:
> On Thu, May 09, 2013 at 05:07:44PM +0200, Peter Zijlstra wrote:
> > On Thu, May 09, 2013 at 03:32:15PM +0200, Jiri Olsa wrote:
> > > hi,
> > > adding sysfs attribute to specify the maximum allowed value
> > > for perf_event_attr::precise_ip field.
> > > 
> > > Adding functionality for simple 'p' modifier and 'precise' term
> > > to get the maximum allowed value for perf_event_attr::precise_ip
> > > field.
> > > 
> > 
> > You've seem to lost the part explaining why we want this.. :-)
> 
> well, initially it was an answer when we broke precise event
> monitoring in kernel so I wrote automated test for it (patches 1,2,3)
> https://lkml.org/lkml/2012/12/12/561

But those don't rely on the max thing right?

> having maximum precise enabled with just single 'p' seemed
> like good idea

Doesn't seem like to me; that takes away the possibility to use less.

> next step would be to enable precise automatically for 'cycles'
> (when PEBS is working) asked for by Ingo
> http://marc.info/?l=linux-kernel&m=135929050803963&w=2

Hurm.. I'm of two minds there. As Stephane has been pointing out for ages,
cycles behaves significantly different between regular and PEBS events for some
cases.

Also, you really don't need the max_precise for that either. At worst you'll
have a number of unsuccessful event creations.
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