jlong os::thread_cpu_time(Thread *thread, bool user_sys_cpu_time) {

  {- -------------------------------------------
  (1) proc ファイルシステム (の /proc/<pid>/lwp/<tid>/lwpusage) から CPU 使用時間の情報を取得する.
      ---------------------------------------- -}

      char proc_name[64];
      int count;
      prusage_t prusage;
      jlong lwp_time;
      int fd;

      sprintf(proc_name, "/proc/%d/lwp/%d/lwpusage",
                         getpid(),
                         thread->osthread()->lwp_id());
      fd = ::open(proc_name, O_RDONLY);
      if ( fd == -1 ) return -1;

      do {
        count = ::pread(fd,
                      (void *)&prusage.pr_utime,
                      thr_time_size,
                      thr_time_off);
      } while (count < 0 && errno == EINTR);
      ::close(fd);
      if ( count < 0 ) return -1;

      if (user_sys_cpu_time) {
        // user + system CPU time
        lwp_time = (((jlong)prusage.pr_stime.tv_sec +
                     (jlong)prusage.pr_utime.tv_sec) * (jlong)1000000000) +
                     (jlong)prusage.pr_stime.tv_nsec +
                     (jlong)prusage.pr_utime.tv_nsec;
      } else {
        // user level CPU time only
        lwp_time = ((jlong)prusage.pr_utime.tv_sec * (jlong)1000000000) +
                    (jlong)prusage.pr_utime.tv_nsec;
      }

      return(lwp_time);
    }