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[104.154.179.209]) by smtp.gmail.com with ESMTPSA id e9e14a558f8ab-3a6eac8c5c0sm127745ab.6.2024.11.06.15.45.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 06 Nov 2024 15:45:25 -0800 (PST) From: Benjamin Peterson To: Cc: Benjamin Peterson , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , "Liang, Kan" , linux-perf-users@vger.kernel.org (open list:PERFORMANCE EVENTS SUBSYSTEM), linux-kernel@vger.kernel.org (open list:PERFORMANCE EVENTS SUBSYSTEM) Subject: [PATCH 1/2] perf trace: do not lose last events in a race Date: Wed, 6 Nov 2024 23:45:17 +0000 Message-Id: <20241106234518.115234-1-benjamin@engflow.com> X-Mailer: git-send-email 2.39.5 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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") Signed-off-by: Benjamin Peterson --- 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