scheduler: Only prioritize rAF if it is fast

components/scheduler/renderer/renderer_scheduler_impl.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 "components/scheduler/renderer/renderer_scheduler_impl.h"
 
 #include "base/bind.h"
 #include "base/debug/stack_trace.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ skipping 661 matching lines @@
   }
 
   if (new_policy_duration > base::TimeDelta()) {
     MainThreadOnly().current_policy_expiration_time = now + new_policy_duration;
     delayed_update_policy_runner_.SetDeadline(FROM_HERE, new_policy_duration,
                                               now);
   } else {
     MainThreadOnly().current_policy_expiration_time = base::TimeTicks();
   }
 
+  // Avoid prioritizing main thread compositing (e.g., rAF) if it is extremely
+  // slow, because that can cause starvation in other task sources.
+  bool main_thread_compositing_is_fast =
+      MainThreadOnly().idle_time_estimator.GetExpectedIdleDuration(
+          MainThreadOnly().compositor_frame_interval) >
+      base::TimeDelta::FromSeconds(0);

[alex clarke (OOO till 29th), 2016/05/03 09:47:01]

As discussed off line I'm wondering if this might be a fairer computation:

double min_idle_ratio = 0.2;  // needs a better name
bool main_thread_compositing_is_fast =
    MainThreadOnly().idle_time_estimator.GetExpectedIdleDuration(
         MainThreadOnly().compositor_frame_interval) >
    MainThreadOnly().compositor_frame_interval * min_idle_ratio;

[Sami, 2016/05/03 12:50:00]

Yep, that's probably safer. Done.

+
   Policy new_policy;
   ExpensiveTaskPolicy expensive_task_policy = ExpensiveTaskPolicy::RUN;
   switch (use_case) {
     case UseCase::COMPOSITOR_GESTURE:
       if (touchstart_expected_soon) {
         expensive_task_policy = ExpensiveTaskPolicy::BLOCK;
         new_policy.compositor_queue_policy.priority = TaskQueue::HIGH_PRIORITY;
       } else {
         // What we really want to do is priorize loading tasks, but that doesn't
         // seem to be safe. Instead we do that by proxy by deprioritizing
         // compositor tasks. This should be safe since we've already gone to the
         // pain of fixing ordering issues with them.
         new_policy.compositor_queue_policy.priority =
             TaskQueue::BEST_EFFORT_PRIORITY;
       }
       break;
 
     case UseCase::SYNCHRONIZED_GESTURE:
-      new_policy.compositor_queue_policy.priority = TaskQueue::HIGH_PRIORITY;
+      new_policy.compositor_queue_policy.priority =
+          main_thread_compositing_is_fast ? TaskQueue::HIGH_PRIORITY
+                                          : TaskQueue::NORMAL_PRIORITY;
       if (touchstart_expected_soon) {
         expensive_task_policy = ExpensiveTaskPolicy::BLOCK;
       } else {
         expensive_task_policy = ExpensiveTaskPolicy::THROTTLE;
       }
       break;
 
     case UseCase::MAIN_THREAD_GESTURE:
       // In main thread gestures we don't have perfect knowledge about which
       // things we should be prioritizing, so we don't attempt to block
       // expensive tasks because we don't know whether they were integral to the
       // page's functionality or not.
-      new_policy.compositor_queue_policy.priority = TaskQueue::HIGH_PRIORITY;
+      new_policy.compositor_queue_policy.priority =
+          main_thread_compositing_is_fast ? TaskQueue::HIGH_PRIORITY
+                                          : TaskQueue::NORMAL_PRIORITY;
       break;
 
     case UseCase::TOUCHSTART:
       new_policy.compositor_queue_policy.priority = TaskQueue::HIGH_PRIORITY;
       new_policy.loading_queue_policy.is_enabled = false;
       new_policy.timer_queue_policy.is_enabled = false;
       // NOTE this is a nop due to the above.
       expensive_task_policy = ExpensiveTaskPolicy::BLOCK;
       break;
 
@@ skipping 521 matching lines @@
     }
     MainThreadOnly().have_reported_blocking_intervention_since_navigation =
         true;
     BroadcastConsoleWarning(
         "Deferred long-running timer task(s) to improve scrolling smoothness. "
         "See crbug.com/574343.");
   }
 }
 
 }  // namespace scheduler