linux-kernel - [BUG] perf: sampling with precise=2 broken in 3.18

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Message-ID: <CABPqkBS-iWehgyp0xEkeJLygeTOg7Ci-3jugRNMJ9Medz=2A1w@mail.gmail.com>
Date:	Mon, 15 Dec 2014 23:51:07 -0500
From:	Stephane Eranian <eranian@...gle.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"ak@...ux.intel.com" <ak@...ux.intel.com>,
	Jiri Olsa <jolsa@...hat.com>,
	"Liang, Kan" <kan.liang@...el.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	dave.hansen@...ux.intel.com
Subject: [BUG] perf: sampling with precise=2 broken in 3.18

Hi,

I was running some perf mem test for an upcoming patch when
I realize that precise=2 was broken on 3.18. It seems it never
(or extremely rarely) correct the off-by-one error, when until 3.18-rc4
it was 100% on the same program. So something was introduced
that broke the asm walker in perf_event_intel_ds.c.

Looking at the log of that file, I can see one change that could have
some impact:

Author: Dave Hansen <dave.hansen@...ux.intel.com>
6ba48ff x86: Remove arbitrary instruction size limit in instruction decoder

if I use a kernel without this fix (prior to that commit), then correction
works. Any kernel after fails. I have not investigated why but may you
have an idea.

To reproduce try using perf mem -t load rec my_load_test, then use
perf report to navigate to the assembly view, the samples should be
on load instructions, not on the instructions following them. If you use
perf mem -t load rec -vv you can verify that precise=2. So something
is not working anymore in the instruction decoder that the fixup routine
bails out.

Any clue?
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