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 a1885571351a7a7ca31a3d63e80342743b0d220f..7c452232f3816b5f2766aeb53d4743888316458a 100644
--- a/base/profiler/scoped_tracker.h
+++ b/base/profiler/scoped_tracker.h
@@ -19,6 +19,22 @@ 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

[Mark Mentovai, 2014/11/04 20:53:41]

I had to read this a few times to make maximum sense of it. Instead of “that
task extends,” how about “that task’s lifetime extends?”

[vadimt, 2014/11/04 21:11:14]

Done.

+// 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.
+// 4. Task creation that this class utilizes was highly optimized by jar@.

[Mark Mentovai, 2014/11/04 20:53:41]

Only if jar is OK being called out by name.

[vadimt, 2014/11/04 21:11:14]

Done.

+// 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.

[Mark Mentovai, 2014/11/04 20:53:42]

Make this stronger: “has to be”→“must”.

[vadimt, 2014/11/04 21:11:14]

Done.

+// However, in rare cases it might be added permanently, as an exception.

[Mark Mentovai, 2014/11/04 20:53:42]

This is kind of wishy-washy. If there are cases where you envision exceptions
being made, you should provide guidance here. Otherwise, you should remove this
sentence, which basically leaves it up to future discretion anyway.

[vadimt, 2014/11/04 21:11:14]

Let's remove for now. If I or someone else wants to leave an instrumentation, we
can revisit this rule later.

 class BASE_EXPORT ScopedTracker {
  public:
   ScopedTracker(const Location& location);