rtla: Count missed trace events

Add function collect_missed_events to trace.c to act as a callback for
tracefs_follow_missed_events, summing the number of total missed events
into a new field missing_events of struct trace_instance.

In case record->missed_events is negative, trace->missed_events is set
to UINT64_MAX to signify an unknown number of events was missed.

The callback is activated on initialization of the trace instance.

Cc: John Kacur <[email protected]>
Cc: Luis Goncalves <[email protected]>
Cc: Gabriele Monaco <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Tomas Glozar <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: lenticularis39

tools/tracing/rtla/src/trace.c

@@ -126,6 +126,31 @@ collect_registered_events(struct tep_event *event, struct tep_record *record,
 	return 0;
 }
 
+/*
+ * collect_missed_events - record number of missed events
+ *
+ * If rtla cannot keep up with events generated by tracer, events are going
+ * to fall out of the ring buffer.
+ * Collect how many events were missed so it can be reported to the user.
+ */
+static int
+collect_missed_events(struct tep_event *event, struct tep_record *record,
+		      int cpu, void *context)
+{
+	struct trace_instance *trace = context;
+
+	if (trace->missed_events == UINT64_MAX)
+		return 0;
+
+	if (record->missed_events > 0)
+		trace->missed_events += record->missed_events;
+	else
+		/* Events missed but no data on how many */
+		trace->missed_events = UINT64_MAX;
+
+	return 0;
+}
+
 /*
  * trace_instance_destroy - destroy and free a rtla trace instance
  */
@@ -181,6 +206,15 @@ int trace_instance_init(struct trace_instance *trace, char *tool_name)
 	 */
 	tracefs_trace_off(trace->inst);
 
+	/*
+	 * Collect the number of events missed due to tracefs buffer
+	 * overflow.
+	 */
+	trace->missed_events = 0;
+	tracefs_follow_missed_events(trace->inst,
+				     collect_missed_events,
+				     trace);
+
 	return 0;
 
 out_err:

tools/tracing/rtla/src/trace.h

@@ -17,6 +17,7 @@ struct trace_instance {
 	struct tracefs_instance		*inst;
 	struct tep_handle		*tep;
 	struct trace_seq		*seq;
+	unsigned long long		missed_events;
 };
 
 int trace_instance_init(struct trace_instance *trace, char *tool_name);