Add a mechanism for cancelling begin/end perf counters.

src/modules/systemlib/perf_counter.c

@@ -201,6 +201,8 @@ perf_end(perf_counter_t handle)
 	switch (handle->type) {
 	case PC_ELAPSED: {
 			struct perf_ctr_elapsed *pce = (struct perf_ctr_elapsed *)handle;
+
+			if (pce->time_start != 0) {
 				hrt_abstime elapsed = hrt_absolute_time() - pce->time_start;
 
 				pce->event_count++;
@@ -211,13 +213,38 @@ perf_end(perf_counter_t handle)
 
 				if (pce->time_most < elapsed)
 					pce->time_most = elapsed;
+
+				pce->time_start = 0;
+			}
 		}
+		break;
 
 	default:
 		break;
 	}
 }
 
+void
+perf_cancel(perf_counter_t handle)
+{
+	if (handle == NULL)
+		return;
+
+	switch (handle->type) {
+	case PC_ELAPSED: {
+			struct perf_ctr_elapsed *pce = (struct perf_ctr_elapsed *)handle;
+
+			pce->time_start = 0;
+		}
+		break;
+
+	default:
+		break;
+	}
+}
+
+
+
 void
 perf_reset(perf_counter_t handle)
 {

src/modules/systemlib/perf_counter.h

@@ -92,13 +92,25 @@ __EXPORT extern void		perf_begin(perf_counter_t handle);
  * End a performance event.
  *
  * This call applies to counters that operate over ranges of time; PC_ELAPSED etc.
+ * If a call is made without a corresopnding perf_begin call, or if perf_cancel
+ * has been called subsequently, no change is made to the counter.
  *
  * @param handle		The handle returned from perf_alloc.
  */
 __EXPORT extern void		perf_end(perf_counter_t handle);
 
 /**
- * Reset a performance event.
+ * Cancel a performance event.
+ *
+ * This call applies to counters that operate over ranges of time; PC_ELAPSED etc.
+ * It reverts the effect of a previous perf_begin.
+ *
+ * @param handle		The handle returned from perf_alloc.
+ */
+__EXPORT extern void		perf_cancel(perf_counter_t handle);
+
+/**
+ * Reset a performance counter.
  *
  * This call resets performance counter to initial state
  *