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

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Message-ID: <20130510094053.GF1040@krava.brq.redhat.com>
Date:	Fri, 10 May 2013 11:40:53 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Fri, May 10, 2013 at 11:27:41AM +0200, Peter Zijlstra wrote:
> 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?

nope, but I need a automated way to find out if PEBS
is supported in system

> 
> > 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.

hm, we could have another modifier to get system precise value
'P' maybe.. and keep the 'p' logic

> 
> > 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.

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

thanks,
jirka
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