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

From: Arnaldo Carvalho de Melo
Date: Fri Feb 01 2019 - 03:11:06 EST

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@xxxxxxxxx>
> ---
> 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
> + object file first. Optional clang options can be pased by option
> + '--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@quaco ~]# cat ~/.perfconfig
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@quaco ~]# vim /home/acme/git/perf/tools/perf/examples/bpf/augmented_raw_syscalls.c

# Do some changes, save

[root@quaco ~]# 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 ]
[root@quaco ~]#

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


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