Track how much we miss the idle notification limit

src/heap/heap.cc

Index: src/heap/heap.cc
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index c96ea97f1b026b0079d676112ee24dc51cb0927b..97445a61e69094bb6b2a30c72ba561f704f132d9 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -4290,9 +4290,7 @@ bool Heap::IdleNotification(int idle_time_in_ms) {
   // If incremental marking is off, we do not perform idle notification.
   if (!FLAG_incremental_marking) return true;
   base::ElapsedTimer timer;
-  if (FLAG_trace_idle_notification) {
-    timer.Start();
-  }
+  timer.Start();
   isolate()->counters()->gc_idle_time_allotted_in_ms()->AddSample(
       idle_time_in_ms);
   HistogramTimerScope idle_notification_scope(
@@ -4348,8 +4346,17 @@ bool Heap::IdleNotification(int idle_time_in_ms) {
     case DO_NOTHING:
       break;
   }
+
+  int actual_time_ms = static_cast<int>(timer.Elapsed().InMilliseconds());
+  if (actual_time_ms <= idle_time_in_ms) {
+    isolate()->counters()->gc_idle_time_limit_undershot()->AddSample(
+        idle_time_in_ms - actual_time_ms);
+  } else {
+    isolate()->counters()->gc_idle_time_limit_overshot()->AddSample(
+        actual_time_ms - idle_time_in_ms);
+  }
+
   if (FLAG_trace_idle_notification) {
-    int actual_time_ms = static_cast<int>(timer.Elapsed().InMilliseconds());
     PrintF("Idle notification: requested idle time %d ms, actual time %d ms [",
            idle_time_in_ms, actual_time_ms);
     action.Print();