Watchdog

本文介绍了看门狗定时器的工作原理及其在系统中的作用。包括硬件和软件两种实现方式,详细解释了如何通过定时器避免系统因硬件或软件故障而崩溃。同时,还讨论了在现代系统中如何使用看门狗进行故障检测与恢复。

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A watchdog is a fixed-length counter that enables a system to recover from an unexpected hardware or software catastrophe. Unless the system periodically resets the watchdog timer, the watchdog timer assumes a catastrophe and tries to handle the situation.
In general, there are two kinds of watchdog implementations, hardware watchdog and a software watchdog based on timer interrupt. Both software and hardware watchdogs are used in the system. The software watchdog is not implemented in all of the subsystems, e.g., RPM, TZ, and APSS (Android) do not have software watchdogs, while LPASS and MPSS implement software watchdog.
The hardware watchdog module is a piece of hardware that is used to ensure that the processor is not stuck or overloaded, and consists of a timer that counts down from a predetermined value. If the timer is not reset (also known as a dog_kick or petting the dog or servicing the dog) by the corresponding CPU core, it eventually counts to 0 and triggers a watchdog timeout. It is the responsibility of each CPU core to ensure that it keeps resetting the counter. If it is unable to do so (if the dedicated task is starved or the CPU core is locked up, etc.), it is assumed that the system has gone into a bad state.
Historically, MSM ASICs have used one watchdog timer for the chip system. The modem software was responsible for resetting the watchdog (kicking or petting the dog) and for checking that other processors in the system were functional by doing periodic checks on them (handshaking through interrupt lines). In addition to the reset triggering signal (WatchDog_expired), it is also possible to generate a watchdog interrupt before the watchdog expiration to allow a processor to attempt the recovery of the system before resetting it:

 Bark (FIQ) – Interrupt before the watchdog expires to allow the processer to attempt the recovery of the system before resetting it
  Bite (Reset) – When watchdog timeout happens
The watchdog timer continues counting even after Bark occurs. However, it stops counting upon halting a CPU via JTAG debugger.

### WATCHDOG 在 IT 领域的作用 在 IT 领域,WATCHDOG 是一种用于保障系统稳定性的重要机制。其核心功能在于实时监控系统的运行状态,并在检测到异常情况时采取恢复措施[^2]。具体而言,当操作系统或其他软件组件发生死锁或无响应的情况时,WATCHDOG 能够触发重启操作或者执行其他预定义的动作来恢复正常服务。 #### 实现方式 WATCHDOG 的实现通常依赖于硬件和软件两种形式: 1. **硬件 WATCHDOG** 硬件 WATCHDOG 基于专用芯片设计,能够独立于主机 CPU 运行。它会定期接收来自应用程序的信号(称为“喂狗”动作)。如果一段时间内未收到该信号,则认为系统可能陷入停滞状态并自动触发重置命令[^3]。 2. **软件 WATCHDOG** 软件 WATCHDOG 则完全由程序逻辑构成,在某些嵌入式设备或服务器环境中被广泛采用。开发者可以通过编写定时器函数配合事件监听器完成类似的功能;一旦发现目标进程未能按时更新心跳标志位便会启动应急流程。 以下是基于 Linux 平台下启用软看护进程的一个简单例子: ```python import time import os def watchdog(): while True: try: # Simulate a task that should run periodically. print("Watchdog checking...") # If the main process fails to update this file within timeout period, # an exception will be raised here causing system reboot or other actions. with open("/tmp/heartbeat", 'r') as f: data = f.read() if not data.strip(): raise Exception('Heartbeat lost!') except Exception as e: print(f"Error detected: {e}. Initiating recovery procedure.") break finally: time.sleep(5) if __name__ == "__main__": watchdog() # Start monitoring loop. # In case of failure, perform necessary steps like restarting service etc. os.system("sudo systemctl restart myservice.service") ``` 上述脚本展示了如何构建一个基础版的软件 WATCHDOG 来监视某个特定文件的内容变化作为健康指标之一。如果有任何错误抛出,则立即停止循环并通过调用外部工具重新激活关联的服务实例[^1]。
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