Luckily for us, the Broadcom BCM2835 SoC on the Raspberry Pi comes with a hardware-based watchdog timer that can do just that. You will find this specially useful if you have a Raspberry Pi in a remote location and the operating system hangs and there's no one around to reboot it.
Load the bcm2708_wdog kernel module
To load the watchdog kernel module right now, issue the following command:$ sudo modprobe bcm2708_wdog
If you are running Raspbian, to load the module the next time the system boots, add a line to your /etc/modules file with "bcm2708_wdog". The -a option makes sure tee appends instead.
$ echo "bcm2708_wdog" | sudo tee -a /etc/modules
If you are running Arch Linux, add a file called "bcm2708_wdog.conf" with the text "bcm2708_wdog" in /etc/modules-load.d/ with the following command:
$ echo "bcm2708_wdog" | sudo tee /etc/modules-load.d/bcm2708_wdog.conf
Install the software watchdog daemon
In Raspbian, run the following command:
$ sudo apt-get install watchdog
But in Arch we use pacman:
$ sudo pacman -S watchdog
Then, make sure it runs after every boot.
In Raspbian, run:
$ sudo update-rc.d watchdog defaults
$ sudo chkconfig --add watchdogIn Arch run the following:
$ sudo systemctl enable watchdog
Configure the watchdog daemon
Open /etc/watchdog.conf with your favorite editor (mine is nano).
$ sudo nano /etc/watchdog.conf
Uncomment the line that starts with #watchdog-device by removing the hash (#) to enable the watchdog daemon to use the watchdog device.
Uncomment the line that says #max-load-1 = 24 by removing the hash symbol to reboot the device if the load goes over 24 over 1 minute. A load of 25 of one minute means that you would have needed 25 Raspberry Pis to complete that task in 1 minute. You may tweak this value to your liking.
Start the watchdog daemonIn Raspbian:
$ sudo chkconfig watchdog on
$ sudo /etc/init.d/watchdog start
$ sudo systemctl start watchdog.service
That's it!You are done! You may play around with the settings in /etc/watchdog.conf if you'd like.
The watchdog daemon performs other tests that you will probably want to configure.
Arch Linux users: I'm well aware that the watchdog daemon is not necessary in Arch because you can enable watchdog features with systemd by editing /etc/systemd/system.conf but I prefer the watchdog daemon as it is much more featured.
2013-01-17 11:45 AM EST: Fixed the chkconfig command to start and enable the watchdog daemon at startup.
Source: Raspberry Pi @ Gadgetoid