Instrumenting URLRequestJob::NotifyHeadersComplete to locate the source of jankiness

base/profiler/scoped_tracker.h

Index: base/profiler/scoped_tracker.h
diff --git a/base/profiler/scoped_tracker.h b/base/profiler/scoped_tracker.h
index fbd73099bcf122f59b2d639b381939c177120a0c..8cfee343144335919133a16e7a89ab0db1db2d03 100644
--- a/base/profiler/scoped_tracker.h
+++ b/base/profiler/scoped_tracker.h
@@ -18,6 +18,22 @@ namespace tracked_objects {
 // ScopedTracker instruments a region within the code if the instrumentation is
 // enabled. It can be used, for example, to find out if a source of jankiness is
 // inside the instrumented code region.
+// Details:
+// 1. This class creates a task (like ones created by PostTask calls or IPC
+// message handlers). This task can be seen in chrome://profiler and is sent as
+// a part of profiler data to the UMA server. See profiler_event.proto.
+// 2. That task extends from the object's constructor to its destructor
+// executions.
+// 3. The execution time associated with the task is the wallclock time between
+// its constructor and destructor, minus wallclock times of directly nested
+// tasks.

[mmenke, 2014/11/03 16:19:15]

Ahh...This is the important detail I was really concerned about.

+// 4. Task creation that this class utilizes was highly optimized by jar@.
+// 5. The class doesn't create a task unless this was enabled for the current
+// process. Search for ScopedTracker::Enable for the current list of processes
+// and channels where it's activated.
+// 6. The class is designed for temporarily instrumenting code to find
+// performance problems, after which the instrumentation has to be removed.
+// However, in rare cases it might be added permanently, as an exception.
 class BASE_EXPORT ScopedTracker {
  public:
   ScopedTracker(const Location& location);