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SubjectRe: [PATCH] perf symbol: Fix kernel symbol address display
On 4/24/20 8:23 PM, Arnaldo Carvalho de Melo wrote:
> Em Fri, Apr 24, 2020 at 08:01:22PM +0200, Thomas Richter escreveu:
>> On 4/24/20 5:06 PM, Arnaldo Carvalho de Melo wrote:
>>> Em Fri, Apr 24, 2020 at 02:37:01PM +0200, Thomas Richter escreveu:
>>>> On 4/20/20 10:46 PM, Arnaldo Carvalho de Melo wrote:
>>>>> Em Wed, Apr 15, 2020 at 09:07:44AM +0200, Thomas Richter escreveu:
>>>>>> Running commands
>>>>>> ./perf record -e rb0000 -- find .
>>>>>> ./perf report -v
>>>>> Or when pressing 'V' in the TUI.
>>>>>> reveals symbol names and its addresses. There is a mismatch between
>>>>> Yeah, an address that at some point was put there to help with debugging
>>>>> the symbol resolution, IIRC how it looked like when looking at
>>>>> readelf -sW vmlinux
>>>>> Or any other DSO, for instance, for a glibc symbol here:
>>>>> Using 'perf report -s pid,dso,sym' then pressing 'V':
>>>>> 1.55% 20325:perf /usr/lib64/ 0x161825 B [.] __strlen_avx2
>>>>> [acme@five perf]$ readelf -sW /usr/lib64/ | grep strlen_avx2
>>>>> 24371: 0000000000161810 414 FUNC LOCAL DEFAULT 15 __strlen_avx2
>>>>> [acme@five perf]$
>>>>> Can you check if doing in /lib/modules/.../build/vmlinux produces what
>>>>> appears when 'V' is in place?
>>>>> And perhaps we can also show the DSO offset and the rip as it gets laid
>>>>> out in memory in the end? So we have all the informations?
>>>>> - Arnaldo
>>>> Arnaldo,
>>>> having verified this also works in the TUI mode using 'V', are you going to pick
>>>> this patch?
>>> That would be a change in behaviour, the original intent was to show the
>>> value one would get from the ELF symbol table, isn't that the case?
>> I think there is a misunderstanding.
>> Your example above refers to libc, which shows the addresses
>> correctly (on x86 and s390). I was refering to the kernel dso.
>> The issue only shows up on the kernel DSO (vmlinux and kallsyms)
>> When I use the -v option (or 'V' in TUI) I get this
>> 3.55% find /lib/modules/.../build/vmlinux 0xf11ec v [k] check_chain_key
>> ^^^^^^^
>> This address is not correct as compared to readelf on vmlinux file:
>> [root@m35lp76 linux]# readelf -sW vmlinux| fgrep check_chain_key
>> 20698: 00000000001f0c70 486 FUNC LOCAL DEFAULT 1 check_chain_key
>> [root@m35lp76 linux]#
>> The problem are the mapping functions with are applied for the kernel DSO.
>> In dso__process_kernel_symbol() the mapping is adjusted (around line 903)
>> and function hist_entry_iter_add() saves the modified address in the
>> struct hist_entry->ip. This address is used from now on and printed.
>> Let me know and I will double check this and rework the commit message
>> as it was not good enough.
> Yeah, this may be me not understanding something right, I saw this was a
> corner case, that only happens in debug mode (ok, better provide good
> info here...), so I haven't devoted more than cursory attention to this
> and may be making a bad judgement, so I thought that since this doesn't
> seems urgent, better leave it to when I manage to have solid time to
> devote to this or someone else looks at this.
> Sorry, but these have been difficult time for everybody, and I fear, for
> me, I have to prioritise all the time.
> - Arnaldo

Yes, it is not easy these days.
I just wanted to make sure this patch is not lost. I already have been
asked when this will be fixed.

Thomas Richter, Dept 3252, IBM s390 Linux Development, Boeblingen, Germany
Vorsitzender des Aufsichtsrats: Matthias Hartmann
Geschäftsführung: Dirk Wittkopp
Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294

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