Add a sampling tracing to RenderWidget::DoDeferredUpdate

base/debug/trace_event.h

Index: base/debug/trace_event.h
diff --git a/base/debug/trace_event.h b/base/debug/trace_event.h
index aa4cc759b79eeabca051ff795c07455cf56eb3c2..e0d72bd65a0e26549815ac12b81a940f0cdf523f 100644
--- a/base/debug/trace_event.h
+++ b/base/debug/trace_event.h
@@ -299,9 +299,16 @@
 // want the inconsistency/expense of storing two pointers.
 // |thread_bucket| is [0..2] and is used to statically isolate samples in one
 // thread from others.
-#define TRACE_EVENT_SAMPLE_STATE(thread_bucket, category, name)                \
-  TRACE_EVENT_API_ATOMIC_STORE(                                                \
-      TRACE_EVENT_API_THREAD_BUCKET(thread_bucket),                            \
+#define TRACE_EVENT_SAMPLING_STATE0(category, name) \

[nduca, 2013/07/03 05:34:24]

can you provide some explanation on why the rename was done? I dont review
blink, otherwise I would have asked that there as well. I wonder, having not
looked at this API in a few months, why there isn't the word SET in there
somewhere. Also, after not looking for a while, the 0 suffix makes me think this
is arguments, e.g. TRACE_EVENT0. I know its not, so I start wondering should
this just be

TARCE_EVENT_SET_SAMPLEING_STATE_FOR_BUCKET(bucket_number, category, name)

Its long, but its very obvious that way.

[haraken, 2013/07/03 07:33:15]

Makes sense. I renamed it to TARCE_EVENT_SET_SAMPLEING_STATE_FOR_BUCKET. I'll do
the same rename in Blink accordingly.

+    INTERNAL_TRACE_EVENT_SAMPLING_STATE(0, category, name)
+#define TRACE_EVENT_SAMPLING_STATE1(category, name) \
+    INTERNAL_TRACE_EVENT_SAMPLING_STATE(1, category, name)
+#define TRACE_EVENT_SAMPLING_STATE2(category, name) \
+    INTERNAL_TRACE_EVENT_SAMPLING_STATE(2, category, name)
+
+#define INTERNAL_TRACE_EVENT_SAMPLING_STATE(thread_bucket, category, name) \
+  TRACE_EVENT_API_ATOMIC_STORE(                                            \
+      TRACE_EVENT_API_THREAD_BUCKET(thread_bucket),                        \
       reinterpret_cast<TRACE_EVENT_API_ATOMIC_WORD>(category "\0" name));
 
 // Records a single BEGIN event called "name" immediately, with 0, 1 or 2
@@ -763,6 +770,36 @@ TRACE_EVENT_API_CLASS_EXPORT extern TRACE_EVENT_API_ATOMIC_WORD g_trace_state1;
 TRACE_EVENT_API_CLASS_EXPORT extern TRACE_EVENT_API_ATOMIC_WORD g_trace_state2;
 #define TRACE_EVENT_API_THREAD_BUCKET(thread_bucket)                           \
   g_trace_state##thread_bucket
+
+// TraceEventSamplingState0Scope records the current sampling state
+// and sets a new sampling state. When the scope exists, it restores
+// the sampling state having recorded.
+class TraceEventSamplingState0Scope {

[nduca, 2013/07/03 05:34:24]

Can we templatize this so that this works for 1,2 buckets as well?

[haraken, 2013/07/03 07:33:15]

Done.

+ public:
+  TraceEventSamplingState0Scope(const char* category_and_name) {

[nduca, 2013/07/03 05:34:24]

we prefer people not use the scope objects directly --- see how TRACE_EVENT0
calls INTERNAL_xxx_SCOPED, which then instantiates the scoped object itself.

[haraken, 2013/07/03 07:33:15]

Done.

+    previous_state0_ = TraceEventSamplingState0Scope::Current();
+    TraceEventSamplingState0Scope::Set(category_and_name);
+  }
+
+  ~TraceEventSamplingState0Scope() {
+    TraceEventSamplingState0Scope::Set(previous_state0_);
+  }
+
+  static inline const char* Current() {
+    return reinterpret_cast<const char*>(TRACE_EVENT_API_ATOMIC_LOAD(
+      TRACE_EVENT_API_THREAD_BUCKET(0)));
+  }
+
+  static inline void Set(const char* category_and_name) {
+    TRACE_EVENT_API_ATOMIC_STORE(
+      TRACE_EVENT_API_THREAD_BUCKET(0),
+      reinterpret_cast<TRACE_EVENT_API_ATOMIC_WORD>(
+        const_cast<char*>(category_and_name)));
+  }
+
+ private:
+  const char* previous_state0_;
+};
 #endif
 
 ////////////////////////////////////////////////////////////////////////////////