Telemetry: Use mimimum tracing for timeline benchmarks.

base/message_loop/message_loop.cc

 // Copyright 2013 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.
 
 #include "base/message_loop/message_loop.h"
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/compiler_specific.h"
@@ skipping 394 matching lines @@
     return false;
 
   PendingTask pending_task = deferred_non_nestable_work_queue_.front();
   deferred_non_nestable_work_queue_.pop();
 
   RunTask(pending_task);
   return true;
 }
 
 void MessageLoop::RunTask(const PendingTask& pending_task) {
+  // For absolutely minimal tracing, "poll_cpu" captures _everything_
+  // on a typical message loop thread, so long as the thread runs semi-regular
+  // tasks (for sufficient measurement granularity).
+  TRACE_EVENT_END0(TRACE_DISABLED_BY_DEFAULT("poll_cpu"), "Poll");
+  TRACE_EVENT_BEGIN0(TRACE_DISABLED_BY_DEFAULT("poll_cpu"), "Poll");
+
   tracked_objects::TrackedTime start_time =
       tracked_objects::ThreadData::NowForStartOfRun(pending_task.birth_tally);
 
   TRACE_EVENT_FLOW_END1(TRACE_DISABLED_BY_DEFAULT("toplevel.flow"),
       "MessageLoop::PostTask", TRACE_ID_MANGLE(GetTaskTraceID(pending_task)),
       "queue_duration",
       (start_time - pending_task.EffectiveTimePosted()).InMilliseconds());
   // When tracing memory for posted tasks it's more valuable to attribute the
   // memory allocations to the source function than generically to "RunTask".
   TRACE_EVENT_WITH_MEMORY_TAG2(
@@ skipping 293 matching lines @@
       fd,
       persistent,
       mode,
       controller,
       delegate);
 }
 
 #endif
 
 }  // namespace base