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

[Namhyung Kim]

Hi Stephane,

On Thu, 4 Sep 2014 16:37:51 +0200, Stephane Eranian wrote:
> On Tue, Aug 26, 2014 at 8:38 AM, Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>>
>> 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
>>
> There is something I don't understand in your example above.  The -H
> option shows only DSO with samples. So why do you get the buildid
> without -H and you get no buildid with -H? In other words, I don't
> connect the dots between what -H does on the buildid change for the
> kernel. Looks like you have the buildid in the perf.data file.

Without -H, it just prints all DSOs found in build-id table (rebuilt
during read perf data file header) and skips processing events.  But
with -H, it'd process the event records and so set kernel map to
'[kernel.kallsyms]' - since the kernel mmap event always has the name -
and mark it as hit.  Thus the actual vmlinux can't be marked and then
cannot be printed.

Hmm.. now I'm curious that why the -H option is needed at all.. the perf
record already wrote build-ids that are actually hits..

Thanks,
Namhyung
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