Reland - Refactor signaling in RWP

content/renderer/raster_worker_pool.h

 // Copyright 2015 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.
 
 #ifndef CONTENT_RENDERER_RASTER_WORKER_POOL_H_
 #define CONTENT_RENDERER_RASTER_WORKER_POOL_H_
 
 #include "base/callback.h"
 #include "base/containers/hash_tables.h"
 #include "base/macros.h"
@@ skipping 28 matching lines @@
                        base::TimeDelta delay) override;
   bool RunsTasksOnCurrentThread() const override;
 
   // Overridden from cc::TaskGraphRunner:
   cc::NamespaceToken GetNamespaceToken() override;
   void ScheduleTasks(cc::NamespaceToken token, cc::TaskGraph* graph) override;
   void WaitForTasksToFinishRunning(cc::NamespaceToken token) override;
   void CollectCompletedTasks(cc::NamespaceToken token,
                              cc::Task::Vector* completed_tasks) override;
 
-  // Runs a task from one of the provided categories. Categories listed first
-  // have higher priority.
-  void Run(const std::vector<cc::TaskCategory>& categories);
+  // Runs a task from the provided category.
+  void Run(cc::TaskCategory category);
 
   void FlushForTesting();
 
   // Spawn |num_threads| number of threads and start running work on the
   // worker threads.
   void Start(int num_threads,
              const base::SimpleThread::Options& thread_options);
 
   // Finish running all the posted tasks (and nested task posted by those tasks)
   // of all the associated task runners.
   // Once all the tasks are executed the method blocks until the threads are
   // terminated.
   void Shutdown();
 
   cc::TaskGraphRunner* GetTaskGraphRunner() { return this; }
 
   // Create a new sequenced task graph runner.
   scoped_refptr<base::SequencedTaskRunner> CreateSequencedTaskRunner();
 
  protected:
   ~RasterWorkerPool() override;
 
  private:
   class RasterWorkerPoolSequencedTaskRunner;
   friend class RasterWorkerPoolSequencedTaskRunner;
 
-  // Runs a task from one of the provided categories. Categories listed first
-  // have higher priority. Returns false if there were no tasks to run.
-  bool RunTaskWithLockAcquired(const std::vector<cc::TaskCategory>& categories);
-
-  // Run next task for the given category. Caller must acquire |lock_| prior to
-  // calling this function and make sure at least one task is ready to run.
-  void RunTaskInCategoryWithLockAcquired(cc::TaskCategory category);
-
   // Simple Task for the TaskGraphRunner that wraps a closure.
   // This class is used to schedule TaskRunner tasks on the
   // |task_graph_runner_|.
   class ClosureTask : public cc::Task {
    public:
     explicit ClosureTask(const base::Closure& closure);
 
     // Overridden from cc::Task:
     void RunOnWorkerThread() override;
 
    protected:
     ~ClosureTask() override;
 
    private:
     base::Closure closure_;
 
     DISALLOW_COPY_AND_ASSIGN(ClosureTask);
   };
 
+  // Helper class used to increment the running task count while a task is
+  // running. Automatically decrements when this class goes out of scope.
+  class ScopedTaskCount {
+   public:
+    ScopedTaskCount(int* running_task_count)
+        : running_task_count_(running_task_count) {
+      (*running_task_count_)++;
+    }
+    ~ScopedTaskCount() {
+      (*running_task_count_)--;
+      DCHECK_GE(*running_task_count_, 0);
+    }
+
+   private:
+    int* const running_task_count_;
+  };
+
+  // Run next task for the given category. Caller must acquire |lock_| prior to
+  // calling this function and make sure at least one task is ready to run.
+  void RunTaskWithLockAcquired(cc::TaskCategory category);
+
+  // Helper function which signals CVs for each category where tasks are ready
+  // to be run.
+  void SignalTaskReadyConditionVariables();
+
+  // Helper which returns a ScopedTaskCount for the given category. This
+  // increments the count immediately, and decrements it when the returned
+  // ScopedTaskCount is deleted.
+  ScopedTaskCount ScopedTaskCountForCategory(cc::TaskCategory category);
+
+  // Determines if we should run a new task for the given category. This factors
+  // in whether a task is available and whether the count of running tasks is
+  // low enough to start a new one.
+  bool ShouldRunTaskForCategory(cc::TaskCategory category);
+
+  // Helper which returns the condition variable being used to signal work
+  // available for the given category.
+  base::ConditionVariable& ConditionVariableForCategory(
+      cc::TaskCategory category);
+
   void ScheduleTasksWithLockAcquired(cc::NamespaceToken token,
                                      cc::TaskGraph* graph);
   void CollectCompletedTasksWithLockAcquired(cc::NamespaceToken token,
                                              cc::Task::Vector* completed_tasks);
 
   // The actual threads where work is done.
   std::vector<scoped_ptr<base::SimpleThread>> threads_;
 
   // Lock to exclusively access all the following members that are used to
   // implement the TaskRunner and TaskGraphRunner interfaces.
   base::Lock lock_;
   // Stores the tasks to be run, sorted by priority.
   cc::TaskGraphWorkQueue work_queue_;
   // Namespace used to schedule tasks in the task graph runner.
   cc::NamespaceToken namespace_token_;
   // List of tasks currently queued up for execution.
   cc::Task::Vector tasks_;
   // Graph object used for scheduling tasks.
   cc::TaskGraph graph_;
   // Cached vector to avoid allocation when getting the list of complete
   // tasks.
   cc::Task::Vector completed_tasks_;
-  // Condition variable that is waited on by Run() until new tasks are ready to
-  // run or shutdown starts.
-  base::ConditionVariable has_ready_to_run_tasks_cv_;
+  // Condition variable for each cc::TaskCategory. Waited on by Run() until new
+  // tasks are ready to run or shutdown starts.
+  base::ConditionVariable nonconcurrent_foreground_has_ready_to_run_tasks_cv_;
+  base::ConditionVariable foreground_has_ready_to_run_tasks_cv_;
+  base::ConditionVariable background_has_ready_to_run_tasks_cv_;
+  // Counters which track the number of running tasks for each category.

[reveman, 2016/02/05 17:15:02]

Can we use the existing run count in work_queue instead of duplicating the state
here?

+  int nonconcurrent_foreground_running_task_count_;
+  int foreground_running_task_count_;
+  int background_running_task_count_;
+  // The target number of threads that should be active at any given time.
+  int target_running_task_count_;

[reveman, 2016/02/05 17:15:02]

I find this a bit awkward. Now that we're using the "run count" to determine
what to run, could we just have "num_threads" of foreground priority threads
that can all run nonconcurrent and normal foreground tasks but if a
nonconcurrent task can run or not depends on 0 nonconcurrent tasks currently
running?

In this case only two CVs would be needed, one for background-tasks-can-run and
one for foreground-tasks-can-run. When any foreground thread wakes up, it would
run a nonconcurrent task if possible, otherwise a normal foreground task.

   // Condition variable that is waited on by origin threads until a namespace
   // has finished running all associated tasks.
   base::ConditionVariable has_namespaces_with_finished_running_tasks_cv_;
   // Set during shutdown. Tells Run() to return when no more tasks are pending.
   bool shutdown_;
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_RASTER_WORKER_POOL_H_