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

[Namhyung Kim]

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. :)

Thanks,
Namhyung

>
> >       if (!err)
> >               *end = addr;
> >
>