Adds the concept of Policies to the Blink Scheduler

Source/platform/scheduler/Scheduler.cpp

 // 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 "config.h"
 #include "platform/scheduler/Scheduler.h"
 
 #include "platform/PlatformThreadData.h"
 #include "platform/Task.h"
 #include "platform/ThreadTimers.h"
 #include "platform/TraceEvent.h"
 #include "public/platform/Platform.h"
 #include "wtf/MainThread.h"
 #include "wtf/ThreadingPrimitives.h"
 
 
 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 {
 public:
     MainThreadIdleTaskAdapter(const Scheduler::IdleTask& idleTask, double allottedTimeMs, const TraceLocation& location)
         : m_idleTask(idleTask)
         , m_allottedTimeMs(allottedTimeMs)
         , m_location(location)
     {
     }
@@ skipping 25 matching lines @@
     }
 
     // WebThread::Task implementation.
     virtual void run() OVERRIDE
     {
         Scheduler* scheduler = Scheduler::shared();
         // FIXME: This check should't be necessary, tasks should not outlive blink.
         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();
     }
 };
 
 
 // Can be created from any thread.
 // Note if the scheduler gets shutdown, this may be run after.
 class Scheduler::MainThreadPendingTaskRunner : public WebThread::Task {
 public:
     MainThreadPendingTaskRunner(
         const Scheduler::Task& task, const TraceLocation& location)
         : m_task(task, location)
     {
         ASSERT(Scheduler::shared());
     }
 
     // WebThread::Task implementation.
     virtual void run() OVERRIDE
     {
         Scheduler* scheduler = Scheduler::shared();
         // 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();
     }
 
     Scheduler::TracedTask m_task;
 };
 
 Scheduler* Scheduler::s_sharedScheduler = nullptr;
 
 void Scheduler::initializeOnMainThread()
 {
     s_sharedScheduler = new Scheduler();
 }
 
 void Scheduler::shutdown()
 {
     delete s_sharedScheduler;
     s_sharedScheduler = nullptr;
 }
 
 Scheduler* Scheduler::shared()
 {
     return s_sharedScheduler;
 }
 
 Scheduler::Scheduler()
     : m_sharedTimerFunction(nullptr)
     , m_mainThread(blink::Platform::current()->currentThread())
+    , m_compositorPriorityPolicyEndTimeSeconds(0)
     , m_highPriorityTaskCount(0)
     , m_highPriorityTaskRunnerPosted(false)
+    , m_schedulerPolicy(Normal)
 {
 }
 
 Scheduler::~Scheduler()
 {
     while (hasPendingHighPriorityWork()) {
         swapQueuesAndRunPendingTasks();
     }
 }
 
@@ skipping 17 matching lines @@
 {
     m_mainThread->postTask(new MainThreadPendingTaskRunner(task, location));
 }
 
 void Scheduler::postInputTask(const TraceLocation& location, const Task& task)
 {
     Locker<Mutex> lock(m_pendingTasksMutex);
     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)
 {
     Locker<Mutex> lock(m_pendingTasksMutex);
     m_pendingHighPriorityTasks.append(TracedTask(task, location));
     atomicIncrement(&m_highPriorityTaskCount);
     maybePostMainThreadPendingHighPriorityTaskRunner();
+    TRACE_COUNTER1(TRACE_DISABLED_BY_DEFAULT("blink.scheduler"), "PendingHighPriorityTasks", m_highPriorityTaskCount);
 }
 
 void Scheduler::maybePostMainThreadPendingHighPriorityTaskRunner()
 {
     ASSERT(m_pendingTasksMutex.locked());
     if (m_highPriorityTaskRunnerPosted)
         return;
     m_mainThread->postTask(new MainThreadPendingHighPriorityTaskRunner());
     m_highPriorityTaskRunnerPosted = true;
 }
 
 void Scheduler::postIdleTask(const TraceLocation& location, const IdleTask& idleTask)
 {
     scheduleIdleTask(location, idleTask);
 }
 
 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()
 {
     ASSERT(isMainThread());
 
     // These locks guard against another thread posting input or compositor tasks while we swap the buffers.
     // 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);
 }
 
 void Scheduler::swapQueuesRunPendingTasksAndAllowHighPriorityTaskRunnerPosting()
 {
     ASSERT(isMainThread());
 
     // These locks guard against another thread posting input or compositor tasks while we swap the buffers.
     // 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();
     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");
     int highPriorityTasksExecuted = 0;
     while (!highPriorityTasks.isEmpty()) {
         highPriorityTasks.takeFirst().run();
         highPriorityTasksExecuted++;
     }
 
     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;
 }
 
 void Scheduler::sharedTimerAdapter()
 {
     shared()->tickSharedTimer();
 }
 
 void Scheduler::setSharedTimerFiredFunction(void (*function)())
 {
     m_sharedTimerFunction = function;
     blink::Platform::current()->setSharedTimerFiredFunction(function ? &Scheduler::sharedTimerAdapter : nullptr);
 }
 
 void Scheduler::setSharedTimerFireInterval(double interval)
 {
     blink::Platform::current()->setSharedTimerFireInterval(interval);
 }
 
 void Scheduler::stopSharedTimer()
 {
     blink::Platform::current()->stopSharedTimer();
 }
 
 bool Scheduler::shouldYieldForHighPriorityWork() const
 {
+    // It's only worthwhile yielding in CompositorPriority mode.
+    if (schedulerPolicy() != CompositorPriority)
+        return false;
+
     return hasPendingHighPriorityWork();
 }
 
 bool Scheduler::hasPendingHighPriorityWork() const
 {
     // This method is expected to be run on the main thread, but the high priority tasks will be posted by
     // other threads. We could use locks here, but this function is (sometimes) called a lot by
     // ThreadTimers::sharedTimerFiredInternal so we decided to use atomics + barrier loads here which
     // should be cheaper.
     // NOTE it's possible the barrier read is overkill here, since delayed yielding isn't a big deal.
     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",
         "src_file", m_location.fileName(),
         "src_func", m_location.functionName());
     m_task();
 }
 
 } // namespace blink