[tracing] Ignore tracing allocations in heap profiler

base/trace_event/heap_profiler_allocation_context_tracker.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_CONTEXT_TRACKER_H_
 #define BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_CONTEXT_TRACKER_H_
 
 #include <vector>
 
 #include "base/atomicops.h"
@@ skipping 34 matching lines @@
 
   // Returns the thread-local instance, creating one if necessary. Returns
   // always a valid instance, unless it is called re-entrantly, in which case
   // returns nullptr in the nested calls.
   static AllocationContextTracker* GetInstanceForCurrentThread();
 
   // Set the thread name in the AllocationContextTracker of the current thread
   // 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);
 
   // Pops a frame from the thread-local pseudo stack.
   void PopPseudoStackFrame(StackFrame frame);
 
   // Push and pop current task's context. A stack is used to support nested
   // tasks and the top of the stack will be used in allocation context.
   void PushCurrentTaskContext(const char* context);
   void PopCurrentTaskContext(const char* context);
@@ skipping 11 matching lines @@
   // The pseudo stack where frames are |TRACE_EVENT| names.
   std::vector<StackFrame> pseudo_stack_;
 
   // The thread name is used as the first entry in the pseudo stack.
   const char* thread_name_;
 
   // Stack of tasks' contexts. Context serves as a different dimension than
   // pseudo stack to cluster allocations.
   std::vector<const char*> task_contexts_;
 
+  uint32_t ignore_scope_depth_;
+
   DISALLOW_COPY_AND_ASSIGN(AllocationContextTracker);
 };
 
 }  // namespace trace_event
 }  // namespace base
 
 #endif  // BASE_TRACE_EVENT_HEAP_PROFILER_ALLOCATION_CONTEXT_TRACKER_H_