scheduler: Maintain a constant enqueue order for every task

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

 // Copyright 2014 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 "platform/scheduler/base/task_queue_selector.h"
 
 #include "base/logging.h"
 #include "base/trace_event/trace_event_argument.h"
 #include "platform/scheduler/base/task_queue_impl.h"
 #include "platform/scheduler/base/work_queue.h"
@@ skipping 152 matching lines @@
         WorkQueue** out_work_queue) const {
   WorkQueue* immediate_queue;
   DCHECK_EQ(*out_chose_delayed_over_immediate, false);
   EnqueueOrder immediate_enqueue_order;
   if (immediate_work_queue_sets_.GetOldestQueueAndEnqueueOrderInSet(
           priority, &immediate_queue, &immediate_enqueue_order)) {
     WorkQueue* delayed_queue;
     EnqueueOrder delayed_enqueue_order;
     if (delayed_work_queue_sets_.GetOldestQueueAndEnqueueOrderInSet(
             priority, &delayed_queue, &delayed_enqueue_order)) {
-      if (immediate_enqueue_order < delayed_enqueue_order) {
+      // When comparing enqueue orders of immediate and delayed tasks, only look
+      // at the sequence number because the delay has already been applied. Note
+      // that the delayed run time is still taken into account by TaskQueueSets
+      // when comparing tasks across different queues.
+      if (immediate_enqueue_order.sequence_num <
+          delayed_enqueue_order.sequence_num) {
         *out_work_queue = immediate_queue;
       } else {
         *out_chose_delayed_over_immediate = true;
         *out_work_queue = delayed_queue;
       }
     } else {
       *out_work_queue = immediate_queue;
     }
     return true;
   }
@@ skipping 186 matching lines @@
 }
 
 void TaskQueueSelector::SetImmediateStarvationCountForTest(
     size_t immediate_starvation_count) {
   immediate_starvation_count_ = immediate_starvation_count;
 }
 
 }  // namespace internal
 }  // namespace scheduler
 }  // namespace blink