[PATCH 2/2] perf record: Synthesize COMM event for a command line workload

[Namhyung Kim <namhyung@kernel.org>]

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.

Acked-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
 tools/perf/builtin-record.c | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index 142eeb341b29..b83373adb9f8 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -469,6 +469,43 @@ static void workload_exec_failed_signal(int signo __maybe_unused,
 	child_finished = 1;
 }
 
+static int synthesize_workload_comm_event(struct perf_evlist *evlist, void *arg)
+{
+	union perf_event *event;
+	struct record *rec = arg;
+	struct machine *machine = &rec->session->machines.host;
+	int pid = evlist->workload.pid;
+	const char *comm_str = program_invocation_short_name;
+	size_t comm_size, total_size;
+	int ret;
+
+	comm_size = PERF_ALIGN(strlen(comm_str) + 1, sizeof(u64));
+	total_size = sizeof(event->comm) + machine->id_hdr_size;
+	/*
+	 * (aligned) comm size might be smaller than expected size
+	 * (i.e.  size of event->comm.comm[]), in that case it needs
+	 * to shrink the total size.
+	 */
+	if (comm_size < sizeof(event->comm.comm))
+		total_size -= sizeof(event->comm.comm) - comm_size;
+
+	event = zalloc(total_size);
+	if (event == NULL)
+		return -ENOMEM;
+
+	event->comm.header.type = PERF_RECORD_COMM;
+	event->comm.header.size = total_size;
+
+	event->comm.pid = pid;
+	event->comm.tid = pid;
+	strncpy(event->comm.comm, comm_str, comm_size);
+
+	ret = record__write(rec, event, total_size);
+
+	free(event);
+	return ret;
+}
+
 static void snapshot_sig_handler(int sig);
 
 static int __cmd_record(struct record *rec, int argc, const char **argv)
@@ -637,7 +674,9 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
 	 * Let the child rip
 	 */
 	if (forks)
-		perf_evlist__start_workload(rec->evlist);
+		perf_evlist__start_workload_ex(rec->evlist,
+					       synthesize_workload_comm_event,
+					       rec);
 
 	if (opts->initial_delay) {
 		usleep(opts->initial_delay * 1000);
-- 
2.5.0