Adds the concept of Policies to the Blink Scheduler

Source/platform/scheduler/Scheduler.cpp

Index: Source/platform/scheduler/Scheduler.cpp
diff --git a/Source/platform/scheduler/Scheduler.cpp b/Source/platform/scheduler/Scheduler.cpp
index 4a447721f58921b8fe0500568ed583f76288b5c8..eb6c698604630c429e1374f82446497987a24b74 100644
--- a/Source/platform/scheduler/Scheduler.cpp
+++ b/Source/platform/scheduler/Scheduler.cpp
@@ -18,6 +18,9 @@ namespace blink {
 
 namespace {
 
+// The time we should stay in CompositorPriority mode for, after a touch event.
+double kLowSchedulerPolicyAfterTouchTimeSeconds = 0.1;
+
 // Can be created from any thread.
 // Note if the scheduler gets shutdown, this may be run after.
 class MainThreadIdleTaskAdapter : public WebThread::Task {
@@ -63,6 +66,8 @@ public:
         ASSERT(scheduler);
         if (!scheduler)
             return;
+        // NOTE we must unconditionally execute high priority tasks here, since if we're not in CompositorPriority
+        // mode, then this is the only place where high priority tasks will be executed.
         scheduler->swapQueuesRunPendingTasksAndAllowHighPriorityTaskRunnerPosting();
     }
 };
@@ -86,7 +91,7 @@ public:
         // FIXME: This check should't be necessary, tasks should not outlive blink.
         ASSERT(scheduler);
         if (scheduler)
-            Scheduler::shared()->swapQueuesAndRunPendingTasks();
+            Scheduler::shared()->runPendingHighPriorityTasksIfInCompositorPriority();
         m_task.run();
     }
 
@@ -114,8 +119,10 @@ Scheduler* Scheduler::shared()
 Scheduler::Scheduler()
     : m_sharedTimerFunction(nullptr)
     , m_mainThread(blink::Platform::current()->currentThread())
+    , m_compositorPriorityPolicyEndTimeSeconds(0)
     , m_highPriorityTaskCount(0)
     , m_highPriorityTaskRunnerPosted(false)
+    , m_schedulerPolicy(Normal)
 {
 }
 
@@ -153,6 +160,7 @@ void Scheduler::postInputTask(const TraceLocation& location, const Task& task)
     m_pendingHighPriorityTasks.append(TracedTask(task, location));
     atomicIncrement(&m_highPriorityTaskCount);
     maybePostMainThreadPendingHighPriorityTaskRunner();
+    TRACE_COUNTER1(TRACE_DISABLED_BY_DEFAULT("blink.scheduler"), "PendingHighPriorityTasks", m_highPriorityTaskCount);
 }
 
 void Scheduler::postCompositorTask(const TraceLocation& location, const Task& task)
@@ -161,6 +169,7 @@ void Scheduler::postCompositorTask(const TraceLocation& location, const Task& ta
     m_pendingHighPriorityTasks.append(TracedTask(task, location));
     atomicIncrement(&m_highPriorityTaskCount);
     maybePostMainThreadPendingHighPriorityTaskRunner();
+    TRACE_COUNTER1(TRACE_DISABLED_BY_DEFAULT("blink.scheduler"), "PendingHighPriorityTasks", m_highPriorityTaskCount);
 }
 
 void Scheduler::maybePostMainThreadPendingHighPriorityTaskRunner()
@@ -182,13 +191,22 @@ void Scheduler::tickSharedTimer()
     TRACE_EVENT0("blink", "Scheduler::tickSharedTimer");
 
     // Run any high priority tasks that are queued up, otherwise the blink timers will yield immediately.
-    bool workDone = swapQueuesAndRunPendingTasks();
+    bool workDone = runPendingHighPriorityTasksIfInCompositorPriority();
     m_sharedTimerFunction();
 
     // The blink timers may have just yielded, so run any high priority tasks that where queued up
     // while the blink timers were executing.
     if (!workDone)
-        swapQueuesAndRunPendingTasks();
+        runPendingHighPriorityTasksIfInCompositorPriority();
+}
+
+bool Scheduler::runPendingHighPriorityTasksIfInCompositorPriority()
+{
+    ASSERT(isMainThread());
+    if (schedulerPolicy() != CompositorPriority)
+        return false;
+
+    return swapQueuesAndRunPendingTasks();
 }
 
 bool Scheduler::swapQueuesAndRunPendingTasks()
@@ -199,6 +217,7 @@ bool Scheduler::swapQueuesAndRunPendingTasks()
     // One the buffers have been swapped we can safely access the returned deque without having to lock.
     m_pendingTasksMutex.lock();
     Deque<TracedTask>& highPriorityTasks = m_pendingHighPriorityTasks.swapBuffers();
+    maybeEnterNormalSchedulerPolicy();
     m_pendingTasksMutex.unlock();
     return executeHighPriorityTasks(highPriorityTasks);
 }
@@ -212,10 +231,21 @@ void Scheduler::swapQueuesRunPendingTasksAndAllowHighPriorityTaskRunnerPosting()
     m_pendingTasksMutex.lock();
     Deque<TracedTask>& highPriorityTasks = m_pendingHighPriorityTasks.swapBuffers();
     m_highPriorityTaskRunnerPosted = false;
+    maybeEnterNormalSchedulerPolicy();
     m_pendingTasksMutex.unlock();
     executeHighPriorityTasks(highPriorityTasks);
 }
 
+void Scheduler::maybeEnterNormalSchedulerPolicy()
+{
+    ASSERT(isMainThread());
+    ASSERT(m_pendingTasksMutex.locked());
+
+    // Go back to the normal scheduler policy if enough time has elapsed.
+    if (schedulerPolicy() == CompositorPriority && Platform::current()->monotonicallyIncreasingTime() > m_compositorPriorityPolicyEndTimeSeconds)
+        enterSchedulerPolicyLocked(Normal);
+}
+
 bool Scheduler::executeHighPriorityTasks(Deque<TracedTask>& highPriorityTasks)
 {
     TRACE_EVENT0("blink", "Scheduler::executeHighPriorityTasks");
@@ -227,6 +257,7 @@ bool Scheduler::executeHighPriorityTasks(Deque<TracedTask>& highPriorityTasks)
 
     int highPriorityTaskCount = atomicSubtract(&m_highPriorityTaskCount, highPriorityTasksExecuted);
     ASSERT_UNUSED(highPriorityTaskCount, highPriorityTaskCount >= 0);
+    TRACE_COUNTER1(TRACE_DISABLED_BY_DEFAULT("blink.scheduler"), "PendingHighPriorityTasks", m_highPriorityTaskCount);
     return highPriorityTasksExecuted > 0;
 }
 
@@ -253,6 +284,10 @@ void Scheduler::stopSharedTimer()
 
 bool Scheduler::shouldYieldForHighPriorityWork() const
 {
+    // It's only worthwhile yielding in CompositorPriority mode.
+    if (schedulerPolicy() != CompositorPriority)
+        return false;
+
     return hasPendingHighPriorityWork();
 }
 
@@ -266,6 +301,31 @@ bool Scheduler::hasPendingHighPriorityWork() const
     return acquireLoad(&m_highPriorityTaskCount) != 0;
 }
 
+Scheduler::SchedulerPolicy Scheduler::schedulerPolicy() const
+{
+    ASSERT(isMainThread());
+    // It's important not to miss the transition from normal to low latency mode, otherwise we're likely to
+    // delay the processing of input tasks. Since that transition is triggered by a different thread, we
+    // need either a lock or a memory barrier, and the memory barrier is probably cheaper.
+    return static_cast<SchedulerPolicy>(acquireLoad(&m_schedulerPolicy));
+}
+
+void Scheduler::enterSchedulerPolicy(SchedulerPolicy schedulerPolicy)
+{
+    Locker<Mutex> lock(m_pendingTasksMutex);
+    enterSchedulerPolicyLocked(schedulerPolicy);
+}
+
+void Scheduler::enterSchedulerPolicyLocked(SchedulerPolicy schedulerPolicy)
+{
+    ASSERT(m_pendingTasksMutex.locked());
+    if (schedulerPolicy == CompositorPriority)
+        m_compositorPriorityPolicyEndTimeSeconds = Platform::current()->monotonicallyIncreasingTime() + kLowSchedulerPolicyAfterTouchTimeSeconds;
+
+    releaseStore(&m_schedulerPolicy, schedulerPolicy);
+    TRACE_COUNTER1(TRACE_DISABLED_BY_DEFAULT("blink.scheduler"), "SchedulerPolicy", schedulerPolicy);
+}
+
 void Scheduler::TracedTask::run()
 {
     TRACE_EVENT2("blink", "TracedTask::run",