[tracing] Ignore tracing allocations in heap profiler

base/trace_event/heap_profiler_allocation_context_tracker.h

Index: base/trace_event/heap_profiler_allocation_context_tracker.h
diff --git a/base/trace_event/heap_profiler_allocation_context_tracker.h b/base/trace_event/heap_profiler_allocation_context_tracker.h
index d6133323bfb3d8f88f4331f6c718582c5611b3ee..7265247efd48e5832279e61cab990bc924baab4d 100644
--- a/base/trace_event/heap_profiler_allocation_context_tracker.h
+++ b/base/trace_event/heap_profiler_allocation_context_tracker.h
@@ -52,6 +52,15 @@ class BASE_EXPORT AllocationContextTracker {
   // if capture is enabled.
   static void SetCurrentThreadName(const char* name);
 
+  // Starts and ends a new ignore scope between which the allocations are
+  // ignored in the heap profiler. A dummy context is returned for these

[Primiano Tucci (use gerrit), 2016/04/21 19:47:31]

I'd say:
A dummy context that short circuits to "tracing_overhead" is returned...

[ssid, 2016/04/22 04:57:12]

Done.

+  // allocations.
+  void begin_ignore_scope() { ignore_scope_depth_++; }
+  void end_ignore_scope() {
+    DCHECK(ignore_scope_depth_);

[Primiano Tucci (use gerrit), 2016/04/21 19:47:31]

thinking more I think you should remove the dcheck and just do

if(ignore_scope_depth)
  ignore_scope_depth--;

I think there is a rare case where you can hit this case legitimately, Imagine
the following sequence:


(tracing is not enabled yet)
Thread 1:
  begin_scoped_ignore()
  do something
Thread 2:
  enable tracing, now capture_enabled == true
Thread 1:
  end_scoped_ignore()

When T1 gets to end_scoped_ignore the depth was 0 because tracing was not
enabled before.
So you might hit the dcheck (and wrap back the depth).
I'd just add the if and leave happy without the dcheck

[ssid, 2016/04/22 04:57:12]

Makes sense. done.

+    ignore_scope_depth_--;
+  }
+
   // Pushes a frame onto the thread-local pseudo stack.
   void PushPseudoStackFrame(StackFrame frame);
 
@@ -83,6 +92,8 @@ class BASE_EXPORT AllocationContextTracker {
   // pseudo stack to cluster allocations.
   std::vector<const char*> task_contexts_;
 
+  uint32_t ignore_scope_depth_;
+
   DISALLOW_COPY_AND_ASSIGN(AllocationContextTracker);
 };