Bug 1347400 - Add telemetry for imgLoader::LoadImage(); r=mystor,francois

image/imgLoader.cpp

@@ -12,6 +12,7 @@
 #include "mozilla/Move.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/ChaosMode.h"
+#include "mozilla/Telemetry.h"
 
 #include "nsImageModule.h"
 #include "imgRequestProxy.h"
@@ -2063,6 +2064,12 @@ imgLoader::LoadImage(nsIURI* aURI,
                      const nsAString& initiatorType,
                      imgRequestProxy** _retval)
 {
+  // Note: We round the time to the nearest milliseconds.  Due to this rounding,
+  // the actual minimum value is 500 microseconds.
+  static const uint32_t kMinTelemetryLatencyMs = 1;
+
+  mozilla::TimeStamp start = TimeStamp::Now();
+
   VerifyCacheSizes();
 
   NS_ASSERTION(aURI, "imgLoader::LoadImage -- NULL URI pointer");
@@ -2298,8 +2305,11 @@ imgLoader::LoadImage(nsIURI* aURI,
     if (!newChannel) {
       proxy->NotifyListener();
     }
+  }
 
-    return rv;
+  uint32_t latencyMs = round((TimeStamp::Now() - start).ToMilliseconds());
+  if (XRE_IsContentProcess() && latencyMs >= kMinTelemetryLatencyMs) {
+    Telemetry::Accumulate(Telemetry::IMAGE_LOAD_TRIGGER_LATENCY_MS, latencyMs);
   }
 
   NS_ASSERTION(*_retval, "imgLoader::LoadImage -- no return value");

toolkit/components/telemetry/Histograms.json

@@ -1482,6 +1482,16 @@
     "n_buckets": 50,
     "description": "GIF image decode speed (Kbytes/sec)"
   },
+  "IMAGE_LOAD_TRIGGER_LATENCY_MS": {
+    "alert_emails": ["ehsan@mozilla.com"],
+    "expires_in_version": "60",
+    "bug_numbers": [1347376],
+    "kind": "exponential",
+    "low": 1,
+    "high": 1000,
+    "n_buckets": 50,
+    "description": "Measures the number of milliseconds we spend inside imgLoader::LoadImage(). Note: only calls that take more than 500 microseconds and happen in the content process are included in this probe."
+  },
   "IMAGE_DECODE_SPEED_PNG": {
     "expires_in_version": "never",
     "kind": "exponential",