linux-kernel - Re: [PATCH] perf, x86: Make cycles:p working on SNB

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Date:	Thu, 24 May 2012 10:59:27 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Namhyung Kim <namhyung.kim@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Paul Mackerras <paulus@...ba.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] perf, x86: Make cycles:p working on SNB

On Thu, May 24, 2012 at 10:07 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2012-05-24 at 10:00 +0200, Stephane Eranian wrote:
>> On Thu, May 24, 2012 at 9:57 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Thu, 2012-05-24 at 09:52 +0200, Peter Zijlstra wrote:
>> >>
>> >> OK, so how about I queue the below and if the u-code updates gets
>> >> released before this hits Linus' tree we'll make it all go away ;-)
>> >>
>> > Also, I suspect the u-code update might make INST_RETIRED.ANY_P
>> > available on SNB again voiding the need to use a different encoding. But
>> > we'll cross that bridge when we'll get to it.
>>
>> The PREC_DIST is by design, it is not a work-around of any sort.
>> It is an attempt at getting a better distribution of samples when sampling
>> with inst_retired.
>
> Oh completely agreed, note that my patch used UOPS_RETIRED.ALL. Only
> Namhyung used PREC_DIST, mostly I suspect because he's not stared at all
> these manuals for long yet ;-)
>
PREC_DIST is only meaningful when used with PEBS. This is where it improves
the sample distribution (compensation some of the PEBS shadow effect).
Otherwise,
you still habe INST_RETIRED:ANY_P.
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