[PATCH v15 5/8] perf record: Read from overwritable ring buffer

[Wang Nan]

overwrite_evt_state is introduced to reflect the state of overwritable
ring buffers. It is a state machine with following states:

                   .________________(forbid)_____________.
                   |                                     V
 NOTREADY -(0)-> RUNNING --(1)--> DATA_PENDING --(2)--> EMPTY
                   ^  ^              |   ^               |
                   |  |__(forbid)____/   |___(forbid)___/|
                   |                                     |
                    \_________________(3)_______________/

 NOTREADY     : Overwritable evlist is not ready
 RUNNING      : Overwritable ring buffers are recording
 DATA_PENDING : We are required to collect overwritable ring buffers
 EMPTY        : We have collected data from those ring buffers.

 (0): Create overwritable evlist
 (1): Pause ring buffers for reading
 (2): Read from ring buffers
 (3): Resume ring buffers for recording

We can't avoid this complexity. Since we deliberately drop records from
overwritable ring buffer, there's no way for us to check remaining from
ring buffer itself (by checking head and old pointers). Therefore, we
need DATA_PENDING and EMPTY state to help us recording what we have done
to the ring buffer.

With the above state machine, this patch improves record__mmap_read_all(),
read from overwritable ring buffer when DATA_PENDING state is observed.

Since the above state machine governs overwritable ring buffer only, if
there's no such ring buffer, it should stop.

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
Signed-off-by: He Kuang <hekuang@xxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
Cc: Zefan Li <lizefan@xxxxxxxxxx>
Cc: Nilay Vaish <nilayvaish@xxxxxxxxx>
Cc: pi3orama@xxxxxxx
---
 tools/perf/builtin-record.c | 158 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 157 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
index ec1f90c..005a121 100644
--- a/tools/perf/builtin-record.c
+++ b/tools/perf/builtin-record.c
@@ -43,6 +43,33 @@
 #include <sys/mman.h>
 #include <asm/bug.h>
 
+/*
+ * State machine of overwrite_evt_state:
+ *
+ *                   .________________(forbid)_____________.
+ *                   |                                     V
+ * NOTREADY -(0)-> RUNNING --(1)--> DATA_PENDING --(2)--> EMPTY
+ *                   ^  ^              |   ^               |
+ *                   |  |__(forbid)____/   |___(forbid)___/|
+ *                   |                                     |
+ *                    \_________________(3)_______________/
+ *
+ * NOTREADY     : Overwritable evlist is not ready
+ * RUNNING      : Overwritable ring buffers are recording
+ * DATA_PENDING : We are required to collect overwritable ring buffers
+ * EMPTY        : We have collected data from those ring buffers.
+ *
+ * (0): Create overwritable evlist
+ * (1): Pause ring buffers for reading
+ * (2): Read from ring buffers
+ * (3): Resume ring buffers for recording
+ */
+enum overwrite_evt_state {
+	OVERWRITE_EVT_NOTREADY,
+	OVERWRITE_EVT_RUNNING,
+	OVERWRITE_EVT_DATA_PENDING,
+	OVERWRITE_EVT_EMPTY,
+};
 
 struct record {
 	struct perf_tool	tool;
@@ -62,6 +89,7 @@ struct record {
 	bool			buildid_all;
 	bool			timestamp_filename;
 	bool			switch_output;
+	enum overwrite_evt_state overwrite_evt_state;
 	unsigned long long	samples;
 };
 
@@ -343,6 +371,96 @@ int auxtrace_record__snapshot_start(struct auxtrace_record *itr __maybe_unused)
 
 #endif
 
+static void
+record__toggle_overwrite_evlist(struct record *rec,
+				enum overwrite_evt_state state)
+{
+	struct perf_evlist *evlist = rec->overwrite_evlist;
+	enum overwrite_evt_state old_state = rec->overwrite_evt_state;
+	enum action {
+		NONE,
+		PAUSE,
+		RESUME,
+	} action = NONE;
+
+	if (!evlist)
+		return;
+
+	switch (old_state) {
+	case OVERWRITE_EVT_RUNNING: {
+		switch (state) {
+		case OVERWRITE_EVT_DATA_PENDING:
+			action = PAUSE;
+			break;
+		case OVERWRITE_EVT_RUNNING:
+		case OVERWRITE_EVT_EMPTY:
+		case OVERWRITE_EVT_NOTREADY:
+		default:
+			goto state_err;
+		}
+		break;
+	}
+	case OVERWRITE_EVT_DATA_PENDING: {
+		switch (state) {
+		case OVERWRITE_EVT_EMPTY:
+			break;
+		case OVERWRITE_EVT_RUNNING:
+		case OVERWRITE_EVT_DATA_PENDING:
+		case OVERWRITE_EVT_NOTREADY:
+		default:
+			goto state_err;
+		}
+		break;
+	}
+	case OVERWRITE_EVT_EMPTY: {
+		switch (state) {
+		case OVERWRITE_EVT_RUNNING:
+			action = RESUME;
+			break;
+		case OVERWRITE_EVT_EMPTY:
+		case OVERWRITE_EVT_DATA_PENDING:
+		case OVERWRITE_EVT_NOTREADY:
+		default:
+			goto state_err;
+		}
+		break;
+	}
+	case OVERWRITE_EVT_NOTREADY: {
+		switch (state) {
+		case OVERWRITE_EVT_RUNNING:
+			if (rec->overwrite_evlist)
+				break;
+			/* Fall through */
+		case OVERWRITE_EVT_DATA_PENDING:
+		case OVERWRITE_EVT_EMPTY:
+		case OVERWRITE_EVT_NOTREADY:
+		default:
+			goto state_err;
+		}
+		break;
+	}
+	default:
+		WARN_ONCE(1, "Shouldn't get there\n");
+	}
+
+	rec->overwrite_evt_state = state;
+
+	switch (action) {
+	case PAUSE:
+		perf_evlist__pause(evlist);
+		break;
+	case RESUME:
+		perf_evlist__resume(evlist);
+		break;
+	case NONE:
+	default:
+		break;
+	}
+
+state_err:
+	return;
+}
+
 static int record__create_overwrite_evlist(struct record *rec)
 {
 	struct perf_evlist *evlist = rec->evlist;
@@ -357,6 +475,7 @@ static int record__create_overwrite_evlist(struct record *rec)
 			if (rec->overwrite_evlist) {
 				rec->overwrite_evlist->backward = true;
 				rec->overwrite_evlist->overwrite = true;
+				record__toggle_overwrite_evlist(rec, OVERWRITE_EVT_RUNNING);
 				return 0;
 			} else
 				return -ENOMEM;
@@ -460,6 +579,7 @@ try_again:
 
 	session->evlist = evlist;
 	perf_session__set_id_hdr_size(session);
+	rec->overwrite_evt_state = OVERWRITE_EVT_NOTREADY;
 out:
 	return rc;
 }
@@ -585,7 +705,13 @@ static int record__mmap_read_all(struct record *rec)
 	if (err)
 		return err;
 
-	return err;
+	if (rec->overwrite_evt_state == OVERWRITE_EVT_DATA_PENDING) {
+		err = record__mmap_read_evlist(rec, rec->overwrite_evlist);
+		if (err)
+			return err;
+		record__toggle_overwrite_evlist(rec, OVERWRITE_EVT_EMPTY);
+	}
+	return 0;
 }
 
 static void record__init_features(struct record *rec)
@@ -980,6 +1106,17 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
 	for (;;) {
 		unsigned long long hits = rec->samples;
 
+		/*
+		 * rec->overwrite_evt_state is possible to be
+		 * OVERWRITE_EVT_EMPTY here: when done == true and
+		 * hits != rec->samples in previous round.
+		 *
+		 * record__toggle_overwrite_evlist ensure we never
+		 * convert OVERWRITE_EVT_EMPTY to OVERWRITE_EVT_DATA_PENDING.
+		 */
+		if (trigger_is_hit(&switch_output_trigger) || done || draining)
+			record__toggle_overwrite_evlist(rec, OVERWRITE_EVT_DATA_PENDING);
+
 		if (record__mmap_read_all(rec) < 0) {
 			trigger_error(&auxtrace_snapshot_trigger);
 			trigger_error(&switch_output_trigger);
@@ -999,8 +1136,27 @@ static int __cmd_record(struct record *rec, int argc, const char **argv)
 		}
 
 		if (trigger_is_hit(&switch_output_trigger)) {
+			/*
+			 * If switch_output_trigger is hit, the data in
+			 * overwritable ring buffer should have been collected,
+			 * so overwrite_evt_state should be set to
+			 * OVERWRITE_EVT_EMPTY.
+			 *
+			 * If SIGUSR2 raise after or during record__mmap_read_all(),
+			 * record__mmap_read_all() didn't collect data from
+			 * overwritable ring buffer. Read again.
+			 */
+			if (rec->overwrite_evt_state == OVERWRITE_EVT_RUNNING)
+				continue;
 			trigger_ready(&switch_output_trigger);
 
+			/*
+			 * Reenable events in overwrite ring buffer after
+			 * record__mmap_read_all(): we should have collected
+			 * data from it.
+			 */
+			record__toggle_overwrite_evlist(rec, OVERWRITE_EVT_RUNNING);
+
 			if (!quiet)
 				fprintf(stderr, "[ perf record: dump data: Woken up %ld times ]\n",
 					waking);
-- 
1.8.3.4