Re: [PATCH 3/3] perf machine: Include data symbols in the kernel map

[Adrian Hunter]

On 11/07/23 20:30, Namhyung Kim wrote:
> Hi Adrian,
> 
> On Tue, Jul 11, 2023 at 8:19 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> wrote:
>>
>> On 20/06/23 23:18, Namhyung Kim wrote:
>>> When perf record -d is used, it needs data mmaps to symbolize global data.
>>> But it missed to collect kernel data maps so it cannot symbolize them.
>>> Instead of having a separate map, just increase the kernel map size to
>>> include the data section.
>>>
>>> Probably we can have a separate kernel map for data, but the current
>>> code assumes a single kernel map.  So it'd require more changes in other
>>> places and looks error-prone.  I decided not to go that way for now.
>>>
>>> Also it seems the kernel module size already includes the data section.
>>>
>>> For example, my system has the following.
>>>
>>>   $ grep -e _stext -e _etext -e _edata /proc/kallsyms
>>>   ffffffff99800000 T _stext
>>>   ffffffff9a601ac8 T _etext
>>>   ffffffff9b446a00 D _edata
>>>
>>> Size of the text section is (0x9a601ac8 - 0x99800000 = 0xe01ac8) and
>>> size of the data section is (0x9b446a00 - 0x99800000 = 0x1c46a00).
>>>
>>> Before:
>>>   $ perf record -d true
>>>
>>>   $ perf report -D | grep MMAP | head -1
>>>   0 0 0x460 [0x60]: PERF_RECORD_MMAP -1/0: [0xffffffff99800000(0xe01ac8) @ 0xffffffff99800000]: x [kernel.kallsyms]_text
>>>                                                                ^^^^^^^^
>>>                                                                  here
>>> After:
>>>   $ perf report -D | grep MMAP | head -1
>>>   0 0 0x460 [0x60]: PERF_RECORD_MMAP -1/0: [0xffffffff99800000(0x1c46a00) @ 0xffffffff99800000]: x [kernel.kallsyms]_text
>>>                                                                ^^^^^^^^^
>>>
>>> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
>>> ---
>>>  tools/perf/util/machine.c | 5 ++++-
>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
>>> index ddc0a2130caf..e93a66f6e0b3 100644
>>> --- a/tools/perf/util/machine.c
>>> +++ b/tools/perf/util/machine.c
>>> @@ -1218,7 +1218,10 @@ static int machine__get_running_kernel_start(struct machine *machine,
>>>
>>>       *start = addr;
>>>
>>> -     err = kallsyms__get_function_start(filename, "_etext", &addr);
>>> +     if (machine->has_data_mmap)
>>> +             err = kallsyms__get_symbol_start(filename, "_edata", &addr);
>>> +     else
>>> +             err = kallsyms__get_function_start(filename, "_etext", &addr);
>>
>> What is the downside of just extending it unconditionally?
> 
> I don't know.. maybe some people would argue it needs the
> proper protection bits other than 'x' but this patch also breaks it.
> But as I said, I'm not sure if we really want to change that now.
> 
> That said, we can make it unconditional. :)

Might as well to start with.  Will need a big comment.

Also do we know if all arch's do it like that? Perhaps
need to fallback to _etext if _edata is not found?

> 
> Thanks,
> Namhyung
> 
>>
>>>       if (!err)
>>>               *end = addr;
>>>
>>