Re: [PATCH v5 0/4] perf: Support PERF_SAMPLE_READ with inherit

[Ben Gainey <Ben.Gainey@arm.com>]

Hello folks

I appreciate you're all busy, but any feedback on this one?

Thanks
Ben




On Mon, 2024-04-15 at 09:14 +0100, Ben Gainey wrote:
> This change allows events to use PERF_SAMPLE READ with inherit so
> long
> as PERF_SAMPLE_TID is also 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.
>
> This patch changes `perf_event_alloc` relaxing the restriction
> against
> combining `inherit` with `PERF_SAMPLE_READ` so that the combination
> will be allowed so long as `PERF_SAMPLE_TID` is enabled. It modifies
> sampling so that only the count associated with the active thread is
> recorded into the buffer. It modifies the context switch handling so
> that perf contexts are always switched out if they have this kind of
> event so that the correct per-thread state is maintained. Finally,
> the
> tools are updated to allow perf record to specify this combination
> and
> to correctly decode the sample data.
>
> 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.
>
>
> Changes since v4:
>  - Rebase on v6.9-rc1
>  - Removed the dependency on inherit_stat that was previously assumed
>    necessary as per feedback from Namhyung Kim.
>  - Fixed an incorrect use of zfree instead of free in the tools
> leading
>    to an abort on tool shutdown.
>  - Additional test coverage improvements added to perf test.
>  - Cleaned up the remaining bit of irrelevant change missed between
> v3
>    and v4.
>
> 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
>   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 + PERF_SAMPLE_READ when opening events
>
>  include/linux/perf_event.h                    |  1 +
>  kernel/events/core.c                          | 82 ++++++++++++++---
> --
>  tools/lib/perf/evlist.c                       |  1 +
>  tools/lib/perf/evsel.c                        | 48 +++++++++++
>  tools/lib/perf/include/internal/evsel.h       | 54 +++++++++++-
>  tools/perf/tests/attr/README                  |  2 +
>  .../tests/attr/test-record-group-sampling     | 39 ---------
>  .../tests/attr/test-record-group-sampling1    | 50 +++++++++++
>  .../tests/attr/test-record-group-sampling2    | 60 ++++++++++++++
>  tools/perf/tests/attr/test-record-group2      |  9 +-
>  tools/perf/util/evsel.c                       | 19 ++++-
>  tools/perf/util/evsel.h                       |  1 +
>  tools/perf/util/session.c                     | 11 ++-
>  13 files changed, 306 insertions(+), 71 deletions(-)
>  delete mode 100644 tools/perf/tests/attr/test-record-group-sampling
>  create mode 100644 tools/perf/tests/attr/test-record-group-sampling1
>  create mode 100644 tools/perf/tests/attr/test-record-group-sampling2
>

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