[PATCH v2] perf record: Synthesize COMM event for a command line workload

[Namhyung Kim]

When perf creates a new child to profile, the events are enabled on
exec().  And in this case, it doesn't synthesize any event for the
child since they'll be generated during exec().  But there's an window
between the enabling and the event generation.

It used to be overcome since samples are only in kernel (so we always
have the map) and the comm is overridden by a later COMM event.
However it won't work if events are processed and displayed before the
COMM event overrides like in 'perf script'.  This leads to those early
samples (like native_write_msr_safe) not having a comm but pid (like
':15328').

So it needs to synthesize COMM event for the child explicitly before
enabling so that it can have a correct comm.  But at this time, the
comm will be "perf" since it's not exec-ed yet.

Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
 tools/perf/builtin-record.c | 16 +++++++++++++++-
 tools/perf/util/event.c     |  2 +-
 tools/perf/util/event.h     |  5 +++++
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 142eeb341b29..6d8302d4612f 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -636,8 +636,22 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
 	/*
 	 * Let the child rip
 	 */
-	if (forks)
+	if (forks) {
+		union perf_event event;
+
+		/*
+		 * Some H/W events are generated before COMM event
+		 * which is emitted during exec(), so perf script
+		 * cannot see a correct process name for those events.
+		 * Synthesize COMM event to prevent it.
+		 */
+		perf_event__synthesize_comm(tool, &event,
+					    rec->evlist->workload.pid,
+					    process_synthesized_event,
+					    &session->machines.host);
+
 		perf_evlist__start_workload(rec->evlist);
+	}
 
 	if (opts->initial_delay) {
 		usleep(opts->initial_delay * 1000);
diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
index 497157affc9c..6214ad47d554 100644
--- a/tools/perf/util/event.c
+++ b/tools/perf/util/event.c
@@ -167,7 +167,7 @@ static int perf_event__prepare_comm(union perf_event *event, pid_t pid,
 	return 0;
 }
 
-static pid_t perf_event__synthesize_comm(struct perf_tool *tool,
+pid_t perf_event__synthesize_comm(struct perf_tool *tool,
 					 union perf_event *event, pid_t pid,
 					 perf_event__handler_t process,
 					 struct machine *machine)
diff --git a/tools/perf/util/event.h b/tools/perf/util/event.h
index f729df5e25e6..be5cbc7be889 100644
--- a/tools/perf/util/event.h
+++ b/tools/perf/util/event.h
@@ -478,6 +478,11 @@ int perf_event__synthesize_sample(union perf_event *event, u64 type,
 				  const struct perf_sample *sample,
 				  bool swapped);
 
+pid_t perf_event__synthesize_comm(struct perf_tool *tool,
+				  union perf_event *event, pid_t pid,
+				  perf_event__handler_t process,
+				  struct machine *machine);
+
 int perf_event__synthesize_mmap_events(struct perf_tool *tool,
 				       union perf_event *event,
 				       pid_t pid, pid_t tgid,
-- 
2.5.0

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/