Long tasks: add a trace event for long tasks

third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc

Index: third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc
diff --git a/third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc b/third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc
index d409fcf8f7e411d7bbf53e1ae2e8fde7d75294bf..357c8ed0ae94de775ebcf60f7ef69f953e59eba0 100644
--- a/third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc
+++ b/third_party/WebKit/Source/platform/scheduler/base/task_queue_manager.cc
@@ -18,6 +18,8 @@
 #include "platform/scheduler/base/work_queue.h"
 #include "platform/scheduler/base/work_queue_sets.h"
 
+static const double kLongTaskTraceEventThreshold = 0.05;
+
 namespace blink {
 namespace scheduler {
 
@@ -536,11 +538,11 @@ TaskQueueManager::ProcessTaskResult TaskQueueManager::ProcessTaskFromWorkQueue(
 
   currently_executing_task_queue_ = prev_executing_task_queue;
 
-
+  double task_end_time = 0;
   if (queue->GetShouldNotifyObservers()) {
     if (task_start_time) {
       *time_after_task = real_time_domain()->Now();
-      double task_end_time = MonotonicTimeInSeconds(*time_after_task);
+      task_end_time = MonotonicTimeInSeconds(*time_after_task);
       for (auto& observer : task_time_observers_)
         observer.DidProcessTask(queue, task_start_time, task_end_time);
     }
@@ -550,6 +552,12 @@ TaskQueueManager::ProcessTaskResult TaskQueueManager::ProcessTaskFromWorkQueue(
     queue->NotifyDidProcessTask(pending_task);
   }
 
+  if (task_start_time && task_end_time &&
+      task_end_time - task_start_time > kLongTaskTraceEventThreshold) {
+    TRACE_EVENT2("blink", "LongTask", "start_time", task_start_time, "end_time",

[altimin, 2017/07/07 18:24:17]

This will be shown in the UI as an extremely short event. This seems rather odd
and counterintuitive.

+primiano@ as a tracing owner — do we have better mechanisms that this?

[caseq, 2017/07/07 18:54:44]

+1 to this, I don't think it makes sense in the present form. One way to address
this would be to just emit an instant event, then use custom logic in the UI to
render the existing top-level event differently. Or, better, don't emit anything
at all and just have the logic entirely on the client side -- we already have
trace events for all tasks anyway.

[Liquan (Max) Gu, 2017/07/07 20:01:25]

I have changed to emit an instant event. 

From the issue description, it intends to surface this signal to devtools as
well. If it's implemented on the client side, can it be surfaced to devtools.?

+                 task_end_time);
+  }
+
   return ProcessTaskResult::EXECUTED;
 }