[PATCH] perf data: Fix releasing event_class

[Wang Nan]

A new patch of libbabeltrace [1] reveals a object leak problem in
perf data CTF support: perf code never release event_class which is
allocated in add_event() and stored in evsel's private field.

If libbabeltrace has the above patch applied, leaking event_class
prevent writer being destroied and flushing metadata. For example:

 $ ./perf record ls
 Lowering default frequency rate to 500.
 Please consider tweaking /proc/sys/kernel/perf_event_max_sample_rate.
 perf.data
 [ perf record: Woken up 1 times to write data ]
 [ perf record: Captured and wrote 0.012 MB perf.data (12 samples) ]
 $ ./perf data convert --to-ctf ./out.ctf
 [ perf data convert: Converted 'perf.data' into CTF data './out.ctf' ]
 [ perf data convert: Converted and wrote 0.000 MB (12 samples) ]
 $ cat ./out.ctf/metadata
 $ ls -l  ./out.ctf/metadata
 -rw-r----- 1 w00229757 mm 0 Jan 27 10:49 ./out.ctf/metadata

The correct result should be:
 ...
 $ cat ./out.ctf/metadata
 /* CTF 1.8 */

 trace {
 [SNIP]

 $ ls -l  ./out.ctf/metadata
 -rw-r----- 1 w00229757 mm 2446 Jan 27 10:52 ./out.ctf/metadata

The full story is:

 Patch [1] of babeltrace redesign reference counting scheme. In that
 patch:

  * writer <- trace (bt_ctf_writer_create)
  * trace <- stream_class (bt_ctf_trace_add_stream_class)
  * stream_class <- event_class (bt_ctf_stream_class_add_event_class)
  ('<-' means 'is a parent of')

  Holding of event_class causes reference count of corresponding
  'writer' increases through parent chain. Perf expect 'writer' is
  released (so metadata is flushed) through bt_ctf_writer_put() in
  ctf_writer__cleanup(). However, since it never release event_class,
  the reference of 'writer' won't be reduced, so bt_ctf_writer_put()
  won't lead releasing of writer.

 Before this CTF patch, !(writer <- trace). Even event_class leak,
 writer is able to be released.

[1] https://github.com/efficios/babeltrace/commit/e6a8e8e4744633807083a077ff9f101eb97d9801

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: JÃrÃmie Galarneau <jeremie.galarneau@xxxxxxxxxxxx>
Cc: Zefan Li <lizefan@xxxxxxxxxx>
Cc: pi3orama@xxxxxxx
---
 tools/perf/util/data-convert-bt.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/perf/util/data-convert-bt.c b/tools/perf/util/data-convert-bt.c
index 34cd1e4..e553321 100644
--- a/tools/perf/util/data-convert-bt.c
+++ b/tools/perf/util/data-convert-bt.c
@@ -858,6 +858,19 @@ static int setup_events(struct ctf_writer *cw, struct perf_session *session)
 	return 0;
 }
 
+static void cleanup_events(struct perf_session *session)
+{
+	struct perf_evlist *evlist = session->evlist;
+	struct perf_evsel *evsel;
+
+	evlist__for_each(evlist, evsel) {
+		struct evsel_priv *priv;
+
+		priv = evsel->priv;
+		bt_ctf_event_class_put(priv->event_class);
+	}
+}
+
 static int setup_streams(struct ctf_writer *cw, struct perf_session *session)
 {
 	struct ctf_stream **stream;
@@ -1171,6 +1184,7 @@ int bt_convert__perf2ctf(const char *input, const char *path, bool force)
 		(double) c.events_size / 1024.0 / 1024.0,
 		c.events_count);
 
+	cleanup_events(session);
 	perf_session__delete(session);
 	ctf_writer__cleanup(cw);
 
-- 
1.8.3.4