Track how much we miss the idle notification limit

We can't report negative values using histograms, so we split the data
up into two histograms

BUG=chromium:397026
LOG=n
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/572293002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23981 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/counters.h

@@ -291,9 +291,11 @@ class HistogramTimerScope BASE_EMBEDDED {
 #endif
 };
 
-#define HISTOGRAM_RANGE_LIST(HR) \
-  /* Generic range histograms */ \
-  HR(gc_idle_time_allotted_in_ms, V8.GCIdleTimeAllottedInMS, 0, 10000, 101)
+#define HISTOGRAM_RANGE_LIST(HR)                                              \
+  /* Generic range histograms */                                              \
+  HR(gc_idle_time_allotted_in_ms, V8.GCIdleTimeAllottedInMS, 0, 10000, 101)   \
+  HR(gc_idle_time_limit_overshot, V8.GCIdleTimeLimit.Overshot, 0, 10000, 101) \
+  HR(gc_idle_time_limit_undershot, V8.GCIdleTimeLimi.Undershot, 0, 10000, 101)
 
 #define HISTOGRAM_TIMER_LIST(HT)                             \
   /* Garbage collection timers. */                           \

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();