linux-kernel - Re: [PATCH 06/12] perf/x86-ibs: Precise event sampling with IBS for AMD CPUs

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Message-ID: <20120423095659.GS9747@erda.amd.com>
Date:	Mon, 23 Apr 2012 11:56:59 +0200
From:	Robert Richter <robert.richter@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/12] perf/x86-ibs: Precise event sampling with IBS for
 AMD CPUs

On 14.04.12 12:21:46, Peter Zijlstra wrote:
> On Mon, 2012-04-02 at 20:19 +0200, Robert Richter wrote:
> > + * We map IBS sampling to following precise levels:
> > + *
> > + *  1: RIP taken from IBS sample or (if invalid) from stack
> > + *  2: RIP always taken from IBS sample, samples with an invalid rip
> > + *     are dropped. Thus samples of an event containing two precise
> > + *     modifiers (e.g. r076:pp) only contain (precise) addresses
> > + *     detected with IBS.
> 
> 		/*
> 		 * precise_ip:
> 		 *
> 		 *  0 - SAMPLE_IP can have arbitrary skid
> 		 *  1 - SAMPLE_IP must have constant skid
> 		 *  2 - SAMPLE_IP requested to have 0 skid
> 		 *  3 - SAMPLE_IP must have 0 skid
> 		 *
> 		 *  See also PERF_RECORD_MISC_EXACT_IP
> 		 */
> 
> your 1 doesn't have constant skid. I would suggest only supporting 2 and
> letting userspace drop !PERF_RECORD_MISC_EXACT_IP records if so desired.

Ah, didn't notice the PERF_RECORD_MISC_EXACT_IP flag. Will set this
flag for precise events.

Problem is that this flag is not yet well supported, only perf-top
uses it to count the total number of exact samples. Esp. perf-annotate
and perf-report do not support it, and there are no modifiers to
select precise-only sampling (or is this level 3?).

Both might be useful: You might need only precise-rip samples (perf-
annotate usage), on the other side you want samples with every
clock/ops count overflow (e.g. to get a counting statistic). The
p-modifier specification (see perf-list) is not sufficient to select
both of it.

Another question I have: Isn't precise level 2 a special case of level
1 where the skid is constant and 0? The problem I see is, if people
want to measure precise rip, they simply use r076:p. Level 2 (r076:pp)
is actually better than 1, but they might think not to be able to
sample precise-rip if we throw an error for r076:p. Thus, I would
prefer to also allow level 1.

> That said, mixing the IBS pmu into the regular core pmu isn't exactly
> pretty..

IBS is currently the only way to do precise-rip sampling on amd cpus.
IBS events fit well with its corresponding perfctr events (0x76/
0xc1). So what don't you like with this approach? I will also post IBS
perf tool support where IBS can be directly used.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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