linux-kernel - [PATCH v2 1/3] perf trace: do not lose last events in a race

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Message-Id: <20241107232128.108981-1-benjamin@engflow.com>
Date: Thu,  7 Nov 2024 23:21:26 +0000
From: Benjamin Peterson <benjamin@...flow.com>
To: 
Cc: Benjamin Peterson <benjamin@...flow.com>,
	Howard Chu <howardchu95@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	"Liang, Kan" <kan.liang@...ux.intel.com>,
	linux-perf-users@...r.kernel.org (open list:PERFORMANCE EVENTS SUBSYSTEM),
	linux-kernel@...r.kernel.org (open list:PERFORMANCE EVENTS SUBSYSTEM)
Subject: [PATCH v2 1/3] perf trace: do not lose last events in a race

If a perf trace event selector specifies a maximum number of events to output
(i.e., "/nr=N/" syntax), the event printing handler, trace__event_handler,
disables the event selector after the maximum number events are
printed. Furthermore, trace__event_handler checked if the event selector was
disabled before doing any work. This avoided exceeding the maximum number of
events to print if more events were in the buffer before the selector was
disabled. However, the event selector can be disabled for reasons other than
exceeding the maximum number of events. In particular, when the traced
subprocess exits, the main loop disables all event selectors. This meant the
last events of a traced subprocess might be lost to the printing handler's
short-circuiting logic.

This nondeterministic problem could be seen by running the following many times:

  $ perf trace -e syscalls:sys_enter_exit_group true

trace__event_handler should simply check for exceeding the maximum number of
events to print rather than the state of the event selector.

Fixes: a9c5e6c1e9bff ("perf trace: Introduce per-event maximum number of events property")
Tested-by: Howard Chu <howardchu95@...il.com>
Signed-off-by: Benjamin Peterson <benjamin@...flow.com>
---
 tools/perf/builtin-trace.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
index d3f11b90d025..f6179b13b8b4 100644
--- a/tools/perf/builtin-trace.c
+++ b/tools/perf/builtin-trace.c
@@ -3096,13 +3096,8 @@ static int trace__event_handler(struct trace *trace, struct evsel *evsel,
 {
 	struct thread *thread;
 	int callchain_ret = 0;
-	/*
-	 * Check if we called perf_evsel__disable(evsel) due to, for instance,
-	 * this event's max_events having been hit and this is an entry coming
-	 * from the ring buffer that we should discard, since the max events
-	 * have already been considered/printed.
-	 */
-	if (evsel->disabled)
+
+	if (evsel->nr_events_printed >= evsel->max_events)
 		return 0;

 	thread = machine__findnew_thread(trace->host, sample->pid, sample->tid);
-- 
2.39.5