ring-buffer/tracing: Have iterator acknowledge dropped events

Have the ring_buffer_iterator set a flag if events were dropped as it were
to go and peek at the next event. Have the trace file display this fact if
it happened with a "LOST EVENTS" message.

Link: http://lkml.kernel.org/r/20200317213417.045858900@goodmis.orgSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

include/linux/ring_buffer.h

@@ -138,6 +138,7 @@ ring_buffer_iter_peek(struct ring_buffer_iter *iter, u64 *ts);
 void ring_buffer_iter_advance(struct ring_buffer_iter *iter);
 void ring_buffer_iter_reset(struct ring_buffer_iter *iter);
 int ring_buffer_iter_empty(struct ring_buffer_iter *iter);
+bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter);
 
 unsigned long ring_buffer_size(struct trace_buffer *buffer, int cpu);
 

kernel/trace/ring_buffer.c

@@ -510,6 +510,7 @@ struct ring_buffer_iter {
 	u64				read_stamp;
 	u64				page_stamp;
 	struct ring_buffer_event	*event;
+	int				missed_events;
 };
 
 /**
@@ -1988,6 +1989,7 @@ rb_iter_head_event(struct ring_buffer_iter *iter)
 	iter->page_stamp = iter->read_stamp = iter->head_page->page->time_stamp;
 	iter->head = 0;
 	iter->next_event = 0;
+	iter->missed_events = 1;
 	return NULL;
 }
 
@@ -4191,6 +4193,20 @@ ring_buffer_peek(struct trace_buffer *buffer, int cpu, u64 *ts,
 	return event;
 }
 
+/** ring_buffer_iter_dropped - report if there are dropped events
+ * @iter: The ring buffer iterator
+ *
+ * Returns true if there was dropped events since the last peek.
+ */
+bool ring_buffer_iter_dropped(struct ring_buffer_iter *iter)
+{
+	bool ret = iter->missed_events != 0;
+
+	iter->missed_events = 0;
+	return ret;
+}
+EXPORT_SYMBOL_GPL(ring_buffer_iter_dropped);
+
 /**
  * ring_buffer_iter_peek - peek at the next event to be read
  * @iter: The ring buffer iterator

kernel/trace/trace.c

@@ -3388,11 +3388,15 @@ peek_next_entry(struct trace_iterator *iter, int cpu, u64 *ts,
 	struct ring_buffer_event *event;
 	struct ring_buffer_iter *buf_iter = trace_buffer_iter(iter, cpu);
 
-	if (buf_iter)
+	if (buf_iter) {
 		event = ring_buffer_iter_peek(buf_iter, ts);
-	else
+		if (lost_events)
+			*lost_events = ring_buffer_iter_dropped(buf_iter) ?
+				(unsigned long)-1 : 0;
+	} else {
 		event = ring_buffer_peek(iter->array_buffer->buffer, cpu, ts,
 					 lost_events);
+	}
 
 	if (event) {
 		iter->ent_size = ring_buffer_event_length(event);
@@ -4005,6 +4009,10 @@ enum print_line_t print_trace_line(struct trace_iterator *iter)
 	enum print_line_t ret;
 
 	if (iter->lost_events) {
+		if (iter->lost_events == (unsigned long)-1)
+			trace_seq_printf(&iter->seq, "CPU:%d [LOST EVENTS]\n",
+					 iter->cpu);
+		else
 			trace_seq_printf(&iter->seq, "CPU:%d [LOST %lu EVENTS]\n",
 					 iter->cpu, iter->lost_events);
 		if (trace_seq_has_overflowed(&iter->seq))