Bug 1381777 - Record how long we went over the time budget r=sfink data-r=bsmedberg

js/src/gc/Statistics.cpp

@@ -995,6 +995,11 @@ Statistics::endSlice()
                 if (joinTime.ToMilliseconds() > budget_ms)
                     reportLongestPhaseInMajorGC(slice.parallelTimes, JS_TELEMETRY_GC_SLOW_TASK);
             }
+
+            // Record how long we went over budget.
+            int64_t overrun = sliceTime.ToMicroseconds() - (1000 * budget_ms);
+            if (overrun > 0)
+                runtime->addTelemetry(JS_TELEMETRY_GC_BUDGET_OVERRUN, uint32_t(overrun));
         }
 
         sliceCount_++;

js/src/jsfriendapi.h

@@ -121,11 +121,18 @@ JS_TraceShapeCycleCollectorChildren(JS::CallbackTracer* trc, JS::GCCellPtr shape
 extern JS_FRIEND_API(void)
 JS_TraceObjectGroupCycleCollectorChildren(JS::CallbackTracer* trc, JS::GCCellPtr group);
 
+/*
+ * Telemetry reasons passed to the accumulate telemetry callback.
+ *
+ * It's OK for these enum values to change as they will be mapped to a fixed
+ * member of the mozilla::Telemetry::HistogramID enum by the callback.
+ */
 enum {
     JS_TELEMETRY_GC_REASON,
     JS_TELEMETRY_GC_IS_ZONE_GC,
     JS_TELEMETRY_GC_MS,
     JS_TELEMETRY_GC_BUDGET_MS,
+    JS_TELEMETRY_GC_BUDGET_OVERRUN,
     JS_TELEMETRY_GC_ANIMATION_MS,
     JS_TELEMETRY_GC_MAX_PAUSE_MS,
     JS_TELEMETRY_GC_MAX_PAUSE_MS_2,

js/xpconnect/src/XPCJSRuntime.cpp

@@ -2554,6 +2554,9 @@ AccumulateTelemetryCallback(int id, uint32_t sample, const char* key)
       case JS_TELEMETRY_GC_BUDGET_MS:
         Telemetry::Accumulate(Telemetry::GC_BUDGET_MS, sample);
         break;
+      case JS_TELEMETRY_GC_BUDGET_OVERRUN:
+        Telemetry::Accumulate(Telemetry::GC_BUDGET_OVERRUN, sample);
+        break;
       case JS_TELEMETRY_GC_ANIMATION_MS:
         Telemetry::Accumulate(Telemetry::GC_ANIMATION_MS, sample);
         break;

toolkit/components/telemetry/Histograms.json

@@ -1199,6 +1199,15 @@
     "n_buckets": 50,
     "description": "Percent of GC slice done during idle time"
   },
+  "GC_BUDGET_OVERRUN": {
+    "record_in_processes": ["main", "content"],
+    "alert_emails": ["dev-telemetry-gc-alerts@mozilla.org", "jcoppeard@mozilla.com"],
+    "expires_in_version": "never",
+    "kind": "exponential",
+    "high": 100000,
+    "n_buckets": 100,
+    "description": "How long a GC slice ran over its budget in microseconds"
+  },
   "GEOLOCATION_ACCURACY_EXPONENTIAL": {
     "record_in_processes": ["main", "content"],
     "expires_in_version": "default",