* necessarily track queues that have no tasks scheduled.
   */
  fun activeQueues(): List<TaskQueue> {
    this.withLock {
      return busyQueues + readyQueues
    }
  }

  fun cancelAll() {
    assertLockHeld()
    for (i in busyQueues.size - 1 downTo 0) {
      busyQueues[i].cancelAllAndDecide()
    }
    for (i in readyQueues.size - 1 downTo 0) {
      val queue = readyQueues[i]