5.4-stable review patch. If anyone has any objections, please let me know.
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From: Benjamin Peterson benjamin@engflow.com
[ Upstream commit 3fd7c36973a250e17a4ee305a31545a9426021f4 ]
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: a9c5e6c1e9bff42c ("perf trace: Introduce per-event maximum number of events property") Signed-off-by: Benjamin Peterson benjamin@engflow.com Tested-by: Howard Chu howardchu95@gmail.com Cc: Adrian Hunter adrian.hunter@intel.com Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Ian Rogers irogers@google.com Cc: Ingo Molnar mingo@redhat.com Cc: Jiri Olsa jolsa@kernel.org Cc: Kan Liang kan.liang@linux.intel.com Cc: Mark Rutland mark.rutland@arm.com Cc: Namhyung Kim namhyung@kernel.org Cc: Peter Zijlstra peterz@infradead.org Link: https://lore.kernel.org/r/20241107232128.108981-1-benjamin@engflow.com Signed-off-by: Arnaldo Carvalho de Melo acme@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 6052eb057821d..745ea920eca98 100644 --- a/tools/perf/builtin-trace.c +++ b/tools/perf/builtin-trace.c @@ -2401,13 +2401,8 @@ 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);