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

From: Ben Gainey
Date: Thu Feb 08 2024 - 08:17:50 EST


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(-)

--
2.43.0