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: a9c5e6c ("perf trace: Introduce per-event maximum number of events property")
Signed-off-by: Benjamin Peterson <[email protected]>
Tested-by: Howard Chu <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: benjaminp

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);