Bug 1545093 - Add telemetry about full GC cycle, including time between slices, r=jonco

Differential Revision: https://phabricator.services.mozilla.com/D27896

--HG--
extra : moz-landing-system : lando

dom/base/nsJSEnvironment.cpp

@@ -168,6 +168,8 @@ static TimeStamp sLastCCEndTime;
 
 static TimeStamp sLastForgetSkippableCycleEndTime;
 
+static TimeStamp sCurrentGCStartTime;
+
 static bool sCCLockedOut;
 static PRTime sCCLockedOutTime;
 
@@ -2173,6 +2175,7 @@ static void DOMGCSliceCallback(JSContext* aCx, JS::GCProgress aProgress,
     case JS::GC_CYCLE_BEGIN: {
       // Prevent cycle collections and shrinking during incremental GC.
       sCCLockedOut = true;
+      sCurrentGCStartTime = TimeStamp::Now();
       break;
     }
 
@@ -2239,6 +2242,8 @@ static void DOMGCSliceCallback(JSContext* aCx, JS::GCProgress aProgress,
         sNeedsFullGC = false;
       }
 
+      Telemetry::Accumulate(Telemetry::GC_IN_PROGRESS_MS,
+                            TimeBetween(sCurrentGCStartTime, TimeStamp::Now()));
       break;
     }
 

toolkit/components/telemetry/Histograms.json

@@ -646,6 +646,16 @@
     "n_buckets": 50,
     "description": "Time spent running JS GC (ms)"
   },
+  "GC_IN_PROGRESS_MS": {
+    "record_in_processes": ["main", "content"],
+    "alert_emails": ["smaug@mozilla.com"],
+    "expires_in_version": "never",
+    "kind": "exponential",
+    "high": 10000,
+    "n_buckets": 50,
+    "bug_numbers": [1545093],
+    "description": "Time from the beginning of the first slice to the end of the last slice (ms)"
+  },
   "GC_BUDGET_MS": {
     "record_in_processes": ["main", "content"],
     "alert_emails": ["dev-telemetry-gc-alerts@mozilla.org"],