code: public long getCumulativeCpuTime() {
      if (JIFFY_LENGTH_IN_MILLIS < 0) {
        return 0;
      }
      long incJiffies = 0;
      for (ProcessInfo p : processTree.values()) {
        if (p != null) {
          incJiffies += p.dtime;
        }
      }
      if (incJiffies * JIFFY_LENGTH_IN_MILLIS < Integer.MAX_VALUE) {
        // Ignore the values that are ridiculous
        cpuTime += incJiffies * JIFFY_LENGTH_IN_MILLIS;
      }
  
      return cpuTime;
    }
docs: Get the CPU time in millisecond used by all the processes in the
  process-tree since the process-tree created
  
  @return cumulative CPU time in millisecond since the process-tree created
  return 0 if it cannot be calculated