我设置signal.setitimer，然后使用os.system()启动一个阻塞命令，想在5s后退出，但是失败了，什么原因？

Question

我使用了定时器signal.setitimer，想要在5s后退出程序，但是没有成功

这是我的代码：

from signal import setitimer, signal, SIGALRM, ITIMER_REAL, SIGKILL
import os

if __name__ == '__main__':
    def _exit(*args):
        print('Timeout & Exit!!!')
        os.kill(os.getpid(), SIGKILL)
    signal(SIGALRM, _exit)
    setitimer(ITIMER_REAL, 5)
    os.system('ping 127.0.0.1')
    setitimer(ITIMER_REAL, 0)
    

这是运行的结果：
PING 127.0.0.1 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.022 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.017 ms
64 bytes from 127.0.0.1: icmp_seq=3 ttl=64 time=0.011 ms
64 bytes from 127.0.0.1: icmp_seq=4 ttl=64 time=0.011 ms
64 bytes from 127.0.0.1: icmp_seq=5 ttl=64 time=0.011 ms
64 bytes from 127.0.0.1: icmp_seq=6 ttl=64 time=0.011 ms
64 bytes from 127.0.0.1: icmp_seq=7 ttl=64 time=0.011 ms
64 bytes from 127.0.0.1: icmp_seq=8 ttl=64 time=0.010 ms
64 bytes from 127.0.0.1: icmp_seq=9 ttl=64 time=0.012 ms
64 bytes from 127.0.0.1: icmp_seq=10 ttl=64 time=0.011 ms
64 bytes from 127.0.0.1: icmp_seq=11 ttl=64 time=0.011 ms
64 bytes from 127.0.0.1: icmp_seq=12 ttl=64 time=0.011 ms
64 bytes from 127.0.0.1: icmp_seq=13 ttl=64 time=0.011 ms

如果我将os.system('ping 127.0.0.1')替换成while True:pass, 程序会在5s后自动退出，谁能帮忙解释下呢？