Re: [PATCH v4 0/4] perf: Support PERF_SAMPLE_READ with inherit_stat

[Namhyung Kim]

On Fri, Mar 22, 2024 at 9:42 AM Ben Gainey <ben.gainey@xxxxxxx> wrote:
>
> This change allows events to use PERF_SAMPLE READ with inherit so long
> as both inherit_stat and PERF_SAMPLE_TID are set.
>
> Currently it is not possible to use PERF_SAMPLE_READ with inherit. This
> restriction assumes the user is interested in collecting aggregate
> statistics as per `perf stat`. It prevents a user from collecting
> per-thread samples using counter groups from a multi-threaded or
> multi-process application, as with `perf record -e '{....}:S'`. Instead
> users must use system-wide mode, or forgo the ability to sample counter
> groups. System-wide mode is often problematic as it requires specific
> permissions (no CAP_PERFMON / root access), or may lead to capture of
> significant amounts of extra data from other processes running on the
> system.
>
> Perf already supports the ability to collect per-thread counts with
> `inherit` via the `inherit_stat` flag. This patch changes

I'm not sure about this part.  IIUC inherit and inherit_stat is not for
per-thread counts, it only supports per-process (including children)
events.


> `perf_event_alloc` relaxing the restriction to combine `inherit` with
> `PERF_SAMPLE_READ` so that the combination will be allowed so long as
> `inherit_stat` and `PERF_SAMPLE_TID` are enabled.

Anyway, does it really need 'inherit_stat'?  I think it's only for
counting use cases (e.g. 'perf stat') not for sampling.

Also technically, it can have PERF_SAMPLE_STREAM_ID instead
of PERF_SAMPLE_TID to distinguish the counter values.

>
> In this configuration stream ids (such as may appear in the read_format
> field of a PERF_RECORD_SAMPLE) are no longer globally unique, rather
> the pair of (stream id, tid) uniquely identify each event. Tools that
> rely on this, for example to calculate a delta between samples, would
> need updating to take this into account. Previously valid event
> configurations (system-wide, no-inherit and so on) where each stream id
> is the identifier are unaffected.

I think you meant PERF_SAMPLE_ID not PERF_SAMPLE_STREAM_ID.
IIUC the stream id is already unique.

Thanks,
Namhyung

>
>
> Changes since v3:
>  - Cleaned up perf test data changes incorrectly included into this
>    series from elsewhere.
>
> Changes since v2:
>  - Rebase on v6.8
>  - Respond to James Clarke's feedback; fixup some typos and move some
>    repeated checks into a helper macro.
>  - Cleaned up checkpatch lints.
>  - Updated perf test; fixed evsel handling so that existing tests pass
>    and added new tests to cover the new behaviour.
>
> Changes since v1:
>  - Rebase on v6.8-rc1
>  - Fixed value written into sample after child exists.
>  - Modified handling of switch-out so that context with these events
>    take the slow path, so that the per-event/per-thread PMU state is
>    correctly switched.
>  - Modified perf tools to support this mode of operation.
>
> Ben Gainey (4):
>   perf: Support PERF_SAMPLE_READ with inherit_stat
>   tools/perf: Track where perf_sample_ids need per-thread periods
>   tools/perf: Correctly calculate sample period for inherited
>     SAMPLE_READ values
>   tools/perf: Allow inherit + inherit_stat + PERF_SAMPLE_READ when
>     opening events
>
>  include/linux/perf_event.h                    |  1 +
>  kernel/events/core.c                          | 62 ++++++++++----
>  tools/lib/perf/evlist.c                       |  1 +
>  tools/lib/perf/evsel.c                        | 48 +++++++++++
>  tools/lib/perf/include/internal/evsel.h       | 55 ++++++++++++-
>  .../test-record-group-sampling-inherit-stat   | 62 ++++++++++++++
>  tools/perf/util/evsel.c                       | 82 ++++++++++++++++++-
>  tools/perf/util/evsel.h                       |  1 +
>  tools/perf/util/session.c                     | 11 ++-
>  9 files changed, 301 insertions(+), 22 deletions(-)
>  create mode 100644 tools/perf/tests/attr/test-record-group-sampling-inherit-stat
>
> --
> 2.44.0
>