Re: [PATCH v2 0/4] perf: Support PERF_SAMPLE_READ with inherit_stat

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

On Thu, 2024-02-08 at 13:10 +0000, Ben Gainey 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
> `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.
>
> 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 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                    | 53 +++++++++++++++++------
> --
>  tools/lib/perf/evlist.c                 |  1 +
>  tools/lib/perf/evsel.c                  | 48 ++++++++++++++++++++++
>  tools/lib/perf/include/internal/evsel.h | 48 +++++++++++++++++++++-
>  tools/perf/util/evsel.c                 | 15 ++++++-
>  tools/perf/util/evsel.h                 |  1 +
>  tools/perf/util/session.c               | 11 +++--
>  8 files changed, 154 insertions(+), 24 deletions(-)
>



Hello all, appreciate everyone is busy. Is there any feedback on these?
I expect they will need rebasing, but before I do that, are you happy
with the approach?

Cheers
Ben
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