core: don't update render statistics if we didn't actually render

Sometimes a scheduled render can end up doing nothing, e.g. if the
damage region is empty. In that case we don't have valid data to
collect and thus shouldn't update the statistics.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/backend.c

@@ -189,6 +189,7 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 			ps->last_schedule_delay = now_us - ps->next_render;
 		}
 	}
+	ps->did_render = true;
 
 	if (ps->backend_data->ops->prepare) {
 		ps->backend_data->ops->prepare(ps->backend_data, &reg_paint);

src/common.h

@@ -255,6 +255,9 @@ typedef struct session {
 	uint64_t last_schedule_delay;
 	/// When do we want our next frame to start rendering.
 	uint64_t next_render;
+	/// Did we actually render the last frame. Sometimes redraw will be scheduled only
+	/// to find out nothing has changed. In which case this will be set to false.
+	bool did_render;
 	/// Whether we can perform frame pacing.
 	bool frame_pacing;
 

src/picom.c

@@ -240,12 +240,15 @@ void schedule_render(session_t *ps, bool triggered_by_vblank) {
 	    (int)(render_time.tv_sec * 1000000L + render_time.tv_nsec / 1000L);
 	if (ps->target_msc == ps->last_msc) {
 		// The frame has just been displayed, record its render time;
-		log_trace("Last render call took: %d (gpu) + %d (cpu) us, "
-		          "last_msc: %" PRIu64,
-		          render_time_us, (int)ps->last_schedule_delay, ps->last_msc);
-		render_statistics_add_render_time_sample(
-		    &ps->render_stats, render_time_us + (int)ps->last_schedule_delay);
+		if (ps->did_render) {
+			log_trace("Last render call took: %d (gpu) + %d (cpu) us, "
+			          "last_msc: %" PRIu64,
+			          render_time_us, (int)ps->last_schedule_delay, ps->last_msc);
+			render_statistics_add_render_time_sample(
+			    &ps->render_stats, render_time_us + (int)ps->last_schedule_delay);
+		}
 		ps->target_msc = 0;
+		ps->did_render = false;
 		ps->last_schedule_delay = 0;
 	}