linux-kernel - Re: [PATCH] perf record: Add doc for bpf event selection

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Date:   Fri, 1 Feb 2019 09:10:50 +0100
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Changbin Du <changbin.du@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Wang Nan <wangnan0@...wei.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf record: Add doc for bpf event selection

Em Fri, Feb 01, 2019 at 03:06:41AM +0000, Changbin Du escreveu:
> Add document for how to pass bpf program with perf.

That is a good start, see some comments below.
 
> Signed-off-by: Changbin Du <changbin.du@...il.com>
> ---
>  tools/perf/Documentation/perf-record.txt | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
> index d232b13ea713..0925d987cad0 100644
> --- a/tools/perf/Documentation/perf-record.txt
> +++ b/tools/perf/Documentation/perf-record.txt
> @@ -88,6 +88,19 @@ OPTIONS
>            If you want to profile write accesses in [0x1000~1008), just set
>            'mem:0x1000/8:w'.
>  
> +        - a bpf source file or object file to select events by a bpf program.

             A source file (ending in .c) or a precompiled object file (ending in .o)
	     selects one or more BPF events.

> +          The bpf program can attach to variant perf events based on section
             The BPFa                      various
> +          names.
> +
> +          When passing '.c', perf searches installed LLVM to compile it into
                                             a
> +          object file first. Optional clang options can be pased by option
           an
> +          '--clang-opt'.
> +
> +          perf record --clang-opt "-DLINUX_VERSION_CODE=0x50000" \
> +                      -e ./tests/bpf-script-example.c
> +
> +          Note: '--clang-opt' must place before '--event'.

Please mention ~/.perfconfig, that needs a section in the
tools/perf/Documentation/perf-config.txt, for instance I have right now:

[root@...co ~]# cat ~/.perfconfig
[llvm]
	dump-obj = true
	clang-opt = -g

The 'clang-opt' is the same as the command line arg you documented
above, dump object may be used to update a .o file, like:

[root@...co ~]# vim /home/acme/git/perf/tools/perf/examples/bpf/augmented_raw_syscalls.c 
 
 # Do some changes, save

[root@...co ~]# perf record -e /home/acme/git/perf/tools/perf/examples/bpf/augmented_raw_syscalls.c sleep 1
LLVM: dumping /home/acme/git/perf/tools/perf/examples/bpf/augmented_raw_syscalls.o
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.023 MB perf.data ]
[root@...co ~]# 

And if you are up to documenting the remaining features, which I'd
encourage you, please take a look at:

tools/perf/util/llvm-utils.c

In the perf_llvm_config function. there are more things that needs
documenting, and doing a:

  git blame tools/perf/util/llvm-utils.c

Helps in getting the needed explanations, for instance, the original
patch adding the .perfconfig section was this:

git show aa61fd05ca796

And it has, for instance, among others:

            # kbuild directory. If not set, use /lib/modules/`uname -r`/build.
            # If set to "" deliberately, skip kernel header auto-detector.
            kbuild-dir = "/path/to/kernel/build"

            # Options passed to 'make' when detecting kernel header options.
            kbuild-opts = "ARCH=x86_64"

Thanks for working on documenting these features!

- Arnaldo


> +
>  	- a group of events surrounded by a pair of brace ("{event1,event2,...}").
>  	  Each event is separated by commas and the group should be quoted to
>  	  prevent the shell interpretation.  You also need to use --group on
> -- 
> 2.17.1

-- 

- Arnaldo