Reduce CPU usage when no isolates need to be profiled.

runtime/vm/thread_interrupter.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_THREAD_INTERRUPTER_H_
 #define VM_THREAD_INTERRUPTER_H_
 
 #include "vm/allocation.h"
 #include "vm/signal_handler.h"
 #include "vm/thread.h"
 
 
 namespace dart {
 
 struct InterruptedThreadState {
   ThreadId tid;
   uintptr_t pc;
   uintptr_t sp;
   uintptr_t fp;
 };
 
-// When a thread is interrupted the thread specific interrupt
-// callback will be invoked at the interrupt period. Each callback is given an
-// InterruptedThreadState and the thread specific data pointer. When inside a
-// thread interrupt callback doing any of the following
+// When a thread is interrupted the thread specific interrupt callback will be
+// invoked. Each callback is given an InterruptedThreadState and the user data
+// pointer. When inside a thread interrupt callback doing any of the following
 // is forbidden:
 //   * Accessing TLS.
 //   * Allocating memory.
 //   * Taking a lock.
 typedef void (*ThreadInterruptCallback)(const InterruptedThreadState& state,
                                         void* data);
 
 // State stored per registered thread.
 class InterruptableThreadState {
  public:
   ThreadId id;
   ThreadInterruptCallback callback;
   void* data;
 };
 
 class ThreadInterrupter : public AllStatic {
  public:
   static void InitOnce();
 
   static void Startup();
   static void Shutdown();
 
   // Delay between interrupts.
   static void SetInterruptPeriod(intptr_t period);
 
+  // Wake up the thread interrupter thread.
+  static void WakeUp();
+
   // Register the currently running thread for interrupts. If the current thread
   // is already registered, callback and data will be updated.
   static InterruptableThreadState* Register(ThreadInterruptCallback callback,
                                             void* data);
   // Unregister the currently running thread for interrupts.
   static void Unregister();
 
   // Get the current thread state. Will create a thread state if one hasn't
   // been allocated.
   static InterruptableThreadState* GetCurrentThreadState();
   // Get the current thread state. Will not create one if one doesn't exist.
   static InterruptableThreadState* CurrentThreadState();
 
   // Interrupt a thread.
   static void InterruptThread(InterruptableThreadState* thread_state);
 
  private:
   static const intptr_t kMaxThreads = 4096;
   static bool initialized_;
   static bool shutdown_;
   static bool thread_running_;
   static ThreadId interrupter_thread_id_;
   static Monitor* monitor_;
   static intptr_t interrupt_period_;
+  static intptr_t current_wait_time_;
   static ThreadLocalKey thread_state_key_;
 
+  static bool InDeepSleep() {
+    return current_wait_time_ == Monitor::kNoTimeout;
+  }
+
   static InterruptableThreadState* _EnsureThreadStateCreated();
   static void UpdateStateObject(ThreadInterruptCallback callback, void* data);
 
   static void SetCurrentThreadState(InterruptableThreadState* state);
 
   static void ThreadMain(uword parameters);
 
   static void InstallSignalHandler();
 
   friend class ThreadInterrupterVisitIsolates;
 };
 
 void ThreadInterruptNoOp(const InterruptedThreadState& state, void* data);
 
 }  // namespace dart
 
 #endif  // VM_THREAD_INTERRUPTER_H_