Re: [PATCH v3 0/5] perf: generate events for BPF metadata

[Namhyung Kim <namhyung@kernel.org>]

Hi Blake,

On Fri, Jun 06, 2025 at 02:52:41PM -0700, Blake Jones wrote:
> Commit ffa915f46193 ("Merge branch 'bpf_metadata'"), from September 2020,
> added support to the kernel, libbpf, and bpftool to treat read-only BPF
> variables that have names starting with 'bpf_metadata_' specially. This
> patch series updates perf to handle these variables similarly, allowing a
> perf.data file to capture relevant information about BPF programs on the
> system being profiled.
> 
> When it encounters a BPF program, it reads the program's maps to find an
> '.rodata' map with 'bpf_metadata_' variables. If it finds one, it extracts
> their values as strings, and creates a new PERF_RECORD_BPF_METADATA
> synthetic event using that data. It does this both for BPF programs that
> were loaded when a 'perf record' starts, as well as for programs that are
> loaded while the profile is running. For the latter case, it stores the
> metadata for the duration of the profile, and then dumps it at the end of
> the profile, where it's in a better context to do so.
> 
> The PERF_RECORD_BPF_METADATA event holds an array of key-value pairs, where
> the key is the variable name (minus the "bpf_metadata_" prefix) and the
> value is the variable's value, formatted as a string. There is one such
> event generated for each BPF subprogram. Generating it per subprogram
> rather than per program allows it to be correlated with PERF_RECORD_KSYMBOL
> events; the metadata event's "prog_name" is designed to be identical to the
> "name" field of a perf_record_ksymbol. This allows specific BPF metadata to
> be associated with each BPF address range in the collection.
> 
> Changes:
> 
> * v2 -> v3:
>   - Split out event collection from event display.
>   - Resync with tmp.perf-tools-next.
>   - Link to v2:
>     https://lore.kernel.org/linux-perf-users/20250605233934.1881839-1-blakejones@google.com/T/#t
> 
> * v1 -> v2:
>   - Split out libbpf change and send it to the bpf tree.
>   - Add feature detection to perf to detect the libbpf change.
>   - Allow the feature to be skipped if the libbpf support is not found.
>   - Add an example of a PERF_RECORD_BPF_METADATA record.
>   - Change calloc() calls to zalloc().
>   - Don't check for NULL before calling free().
>   - Update the perf_event header when it is created, rather than
>     storing the event size and updating it later.
>   - Add a BPF metadata variable (with the perf version) to all
>     perf BPF programs.
>   - Update the selftest to look for the new perf_version variable.
>   - Split out the selftest into its own patch.
>   - Link to v1:
>     https://lore.kernel.org/linux-perf-users/20250521222725.3895192-1-blakejones@google.com/T/#t
> 
> Blake Jones (5):
>   perf: detect support for libbpf's emit_strings option
>   perf: collect BPF metadata from existing BPF programs
>   perf: collect BPF metadata from new programs
>   perf: display the new PERF_RECORD_BPF_METADATA event
>   perf: add test for PERF_RECORD_BPF_METADATA collection

I tried to process your patches but it failed to build like below:

  util/bpf-event.h: In function 'bpf_metadata_free':
  util/bpf-event.h:68:59: error: unused parameter 'metadata' [-Werror=unused-parameter]
     68 | static inline void bpf_metadata_free(struct bpf_metadata *metadata)
        |                                      ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~

It was a simple to fix by adding __maybe_unused but after that I got
another build error so I stopped.

  /usr/bin/ld: /tmp/tmp.N9VJQ2A3pl/perf-in.o: in function `cmd_record':
  (.text+0x191ae): undefined reference to `perf_event__synthesize_final_bpf_metadata'
  collect2: error: ld returned 1 exit status
  make[4]: *** [Makefile.perf:804: /tmp/tmp.N9VJQ2A3pl/perf] Error 1
  make[4]: *** Waiting for unfinished jobs....
  make[3]: *** [Makefile.perf:290: sub-make] Error 2
  make[2]: *** [Makefile:76: all] Error 2
  make[1]: *** [tests/make:341: make_no_libbpf_O] Error 1
  make: *** [Makefile:109: build-test] Error 2

Please run 'make build-test' and send v4.

Thanks,
Namhyung

> 
>  tools/build/Makefile.feature                |   1 +
>  tools/build/feature/Makefile                |   4 +
>  tools/build/feature/test-libbpf-strings.c   |  10 +
>  tools/lib/perf/include/perf/event.h         |  18 +
>  tools/perf/Documentation/perf-check.txt     |   1 +
>  tools/perf/Makefile.config                  |  12 +
>  tools/perf/Makefile.perf                    |   3 +-
>  tools/perf/builtin-check.c                  |   1 +
>  tools/perf/builtin-inject.c                 |   1 +
>  tools/perf/builtin-record.c                 |   8 +
>  tools/perf/builtin-script.c                 |  15 +-
>  tools/perf/tests/shell/test_bpf_metadata.sh |  76 ++++
>  tools/perf/util/bpf-event.c                 | 378 ++++++++++++++++++++
>  tools/perf/util/bpf-event.h                 |  13 +
>  tools/perf/util/bpf_skel/perf_version.h     |  17 +
>  tools/perf/util/env.c                       |  19 +-
>  tools/perf/util/env.h                       |   4 +
>  tools/perf/util/event.c                     |  21 ++
>  tools/perf/util/event.h                     |   1 +
>  tools/perf/util/header.c                    |   1 +
>  tools/perf/util/session.c                   |   4 +
>  tools/perf/util/synthetic-events.h          |   2 +
>  tools/perf/util/tool.c                      |  14 +
>  tools/perf/util/tool.h                      |   3 +-
>  24 files changed, 622 insertions(+), 5 deletions(-)
>  create mode 100644 tools/build/feature/test-libbpf-strings.c
>  create mode 100755 tools/perf/tests/shell/test_bpf_metadata.sh
>  create mode 100644 tools/perf/util/bpf_skel/perf_version.h
> 
> -- 
> 2.50.0.rc0.604.gd4ff7b7c86-goog
>