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

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Date:   Tue, 11 Jul 2023 18:19:35 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Namhyung Kim <namhyung@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>
Cc:     Ian Rogers <irogers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 3/3] perf machine: Include data symbols in the kernel map

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@...nel.org>
> ---
>  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?

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