linux-kernel - Re: [PATCH] perf x86: Change SandyBridge and IvyBrige instructions event to be precise

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Message-ID: <20130220211819.GA14253@krava.redhat.com>
Date:	Wed, 20 Feb 2013 22:18:19 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Ahern <dsahern@...il.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf x86: Change SandyBridge and IvyBrige instructions
 event to be precise

On Wed, Feb 20, 2013 at 12:55:59PM -0800, Andi Kleen wrote:
> Jiri Olsa <jolsa@...hat.com> writes:
> 
> > Changing the default 'instructions' HW event for SandyBridge and
> > IvyBrige to use the available precise 'instructions' event.
> >
> > The precise event is defined in Intel SDM as:
> >
> > INST_RETIRED.ALL - Precise instruction retired event with HW to
> > reduce effect of PEBS shadow in IP distribution
> >
> > for both SandyBridge and IvyBrige micro-archs. It allows to use
> > precise modifier on 'instructions' event on SandyBridge and
> > IvyBrige micro-archs which is not possible now.
> 
> Actually it's possible, you just have to use it explicitely.
> 
> This will break a lot of profiling setups that assume instructions
> count on the fixed counter and use the 4/8 other counters. 
> Also it will give very bad results with perf stat, which
> you don't want to run with pebs.

right, I haven't realized that..

> 
> Even worse with your change the fixed instruction counter is not
> accessible anymore. So pretty much all the standard setups

still could be used explicitly via cpu/event=0xc0/

> that collect it in addition to other events and fill all 
> the counters would break.
> 
> I don't think it should be done by default, but yes explicitely
> it's quite useful.
> 
> My Haswell patchkit (in the "extended version") exposes
> the precise event as "instructions-p" through sysfs.

will check

thanks,
jirka
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