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Message-ID: <87a96f9bci.fsf@sejong.aot.lge.com>
Date: Fri, 05 Sep 2014 09:09:49 +0900
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Ingo Molnar <mingo@...nel.org>,
Paul Mackerras <paulus@...ba.org>,
Namhyung Kim <namhyung.kim@....com>,
LKML <linux-kernel@...r.kernel.org>,
Jiri Olsa <jolsa@...hat.com>, David Ahern <dsahern@...il.com>,
Adrian Hunter <adrian.hunter@...el.com>,
Andi Kleen <andi@...stfloor.org>,
Stephane Eranian <eranian@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH] perf tools: Fix build-id matching on vmlinux
Hi Arnaldo,
On Thu, 4 Sep 2014 11:18:45 -0300, Arnaldo Carvalho de Melo wrote:
> Em Tue, Aug 26, 2014 at 03:38:39PM +0900, Namhyung Kim escreveu:
>> There's a problem on finding correct kernel symbols when perf report
>> runs on a different kernel. Although a part of the problem was solved
>> by the prior commit 0a7e6d1b6844 ("perf tools: Check recorded kernel
>> version when finding vmlinux"), there's a remaining problem still.
>>
>> When perf records samples, it synthesizes the kernel map using
>> machine__mmap_name() and ref_reloc_sym like "[kernel.kallsyms]_text".
>> You can easily see it using 'perf report -D' command.
>>
>> After finishing record, it goes through the recorded events to find
>> maps/dsos actually used. And then record build-id info of them.
>>
>> During this process, it needs to load symbols in a dso and it'd call
>> dso__load_vmlinux() since the default value of the symbol_conf.try_
>> vmlinux_path is true. However it changes dso->long_name to a real
>> path of the vmlinux file (e.g. /lib/modules/3.16.0-rc2+/build/vmlinux)
>> if one is running on a custom kernel.
>>
>> It resulted in that perf report reads the build-id of the vmlinux, but
>> cannot use it since it only knows about the [kernel.kallsyms] map. It
>> then falls back to possible vmlinux paths by using the recorded kernel
>> version (in case of a recent version) or a running kernel silently
>> (which might break the result). I think it's worth going to the
>> stable tree.
>>
>> I can think of a couple of ways to fix it. In this patch, I changed
>> to use the name of "[kernel.kallsyms]" for the kernel build-id event
>> instead of not trying vmlinux paths. This way we can provide maximum
>> info (like annotation) with minimum change IMHO.
>>
>> Before:
>>
>> $ perf record -a usleep 1
>>
>> $ perf buildid-list
>> 00d5ff078efe1d30b8492854f259215fd877ce30 /lib/modules/3.16.0-rc2+/build/vmlinux
>> 78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
>> 4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
>> 1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
>>
>> $ perf buildid-list -H
>> 0000000000000000000000000000000000000000 [kernel.kallsyms]
>> 78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
>> 4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
>> 1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
>> 0000000000000000000000000000000000000000 /tmp/perf-2523.map
>>
>> After:
>>
>> $ perf record -a usleep 1
>>
>> $ perf buildid-list
>> 00d5ff078efe1d30b8492854f259215fd877ce30 [kernel.kallsyms]
>
> We are losing information, namely the pathname for the kernel used, that
> may be useful in analysis.
Right. That's a problem.
>
> Why not make sure that if there is a build-id in the perf.data header,
> then we completely refusing anything that doesn't match the build-id?
> I.e. the name is irrelevant for this purpose, the contents, as keyed by
> the build-id, is what matters.
The perf report rebuilds machine states from the event records only. In
this case, the kernel map was recorded in the name of [kernel.kallsyms]
so it couldn't find the build-id from the table.
Thanks,
Namhyung
>
> - Arnaldo
>
>> 78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
>> 4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
>> 1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
>>
>> $ perf buildid-list -H
>> 00d5ff078efe1d30b8492854f259215fd877ce30 [kernel.kallsyms]
>> 78186287bba77069a056a5ccbeb14b7fd2ca3a4b /usr/lib64/libc-2.17.so
>> 4eadca6cb82e0a85edb87c15b5e3980742514501 /usr/lib64/ld-2.17.so
>> 1e272ca30081e81ef41935a630eb2f4c636798b4 /usr/lib64/dri/swrast_dri.so
>> 0000000000000000000000000000000000000000 /tmp/perf-2523.map
>>
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Namhyung Kim <namhyung@...nel.org>
>> ---
>> tools/perf/util/header.c | 3 +++
>> 1 file changed, 3 insertions(+)
>>
>> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
>> index 158c787ce0c4..5c4093dee467 100644
>> --- a/tools/perf/util/header.c
>> +++ b/tools/perf/util/header.c
>> @@ -263,6 +263,9 @@ static int __dsos__write_buildid_table(struct list_head *head,
>> machine__mmap_name(machine, nm, sizeof(nm));
>> name = nm;
>> name_len = strlen(nm) + 1;
>> + } else if (dso__is_vmlinux(pos)) {
>> + name = pos->name;
>> + name_len = strlen(pos->name) + 1;
>> } else {
>> name = pos->long_name;
>> name_len = pos->long_name_len + 1;
>> --
>> 2.0.0
--
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