linux-kernel - [PATCH v2 0/4] perf: Support PERF_SAMPLE_READ with inherit

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Message-ID: <20240208131050.2406183-1-ben.gainey@arm.com>
Date: Thu,  8 Feb 2024 13:10:46 +0000
From: Ben Gainey <ben.gainey@....com>
To: linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: peterz@...radead.org,
	mingo@...hat.com,
	acme@...nel.org,
	mark.rutland@....com,
	alexander.shishkin@...ux.intel.com,
	jolsa@...nel.org,
	namhyung@...nel.org,
	irogers@...gle.com,
	adrian.hunter@...el.com,
	james.clark@....com,
	Ben Gainey <ben.gainey@....com>
Subject: [PATCH v2 0/4] perf: Support PERF_SAMPLE_READ with inherit_stat

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