From 3fd7c36973a250e17a4ee305a31545a9426021f4 Mon Sep 17 00:00:00 2001 From: Benjamin Peterson Date: Thu, 7 Nov 2024 23:21:26 +0000 Subject: [PATCH] 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: a9c5e6c1e9bff42c ("perf trace: Introduce per-event maximum number of events property") Signed-off-by: Benjamin Peterson Tested-by: Howard Chu Cc: Adrian Hunter Cc: Alexander Shishkin Cc: Ian Rogers Cc: Ingo Molnar Cc: Jiri Olsa Cc: Kan Liang Cc: Mark Rutland Cc: Namhyung Kim Cc: Peter Zijlstra Link: https://lore.kernel.org/r/20241107232128.108981-1-benjamin@engflow.com Signed-off-by: Arnaldo Carvalho de Melo --- 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 14794725669d..ca5c3f5d6c38 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); -- 2.50.1