Re: [PATCH] perf tools: Fix build-id matching on vmlinux

[Namhyung Kim <namhyung@kernel.org>]

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@vger.kernel.org
>> Signed-off-by: Namhyung Kim <namhyung@kernel.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