Enable Watchdog Reset on Raspbian Jessie for Raspberry Pi 3


Many processors come equipped with a watchdog timer which simply counts down
from some set value to zero.  In order to prevent it from reaching zero, 
the system must periodically reset the watchdog timer.  If the timer reache
zero, it is assumed the system has hung.  The watchdog daemon detects this
condition and can be configured to reboot when this occurs, lessening the need
for manual intervention.

Sources:
   https://www.raspberrypi.org/forums/viewtopic.php?t=175432
   https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=147501
   http://blog.ricardoarturocabral.com/2013/01/auto-reboot-hung-raspberry-pi-using-on.html

Here's possibly the simplest way to enable it:

1. Edit /boot/config.txt and add the following line

   dtparam=watchdog=on


2. Create a conf file /etc/modprobe.d/bcm2835-wdt.conf with contents

   options bcm2835_wdt heartbeat=14 nowayout=0


3. In /etc/systemd/system.conf change this line

   #RuntimeWatchdogSec=0

to

   RuntimeWatchdogSec=14


4. Reboot


5. You can look at the kernel ring buffer (message log) to see the
   status of the watchdog timer

pi@pidev:~$ dmesg | grep watchdog
[    0.835133] bcm2835-wdt 3f100000.watchdog: Broadcom BCM2835 watchdog timer
[    3.326624] systemd[1]: Hardware watchdog 'Broadcom BCM2835 Watchdog timer', version 0
[    3.339998] systemd[1]: Set hardware watchdog to 14s.


6. Finally, you can test the watchdog reset by deliberately blowing up
   the system.  Create the following script, in this case,
   called forkbomb.sh:

#!/bin/bash
echo "Starting shell fork bomb"
# prevent swapping to the SD card!
sudo systemctl stop dphys-swapfile.service
# start the bomb
: (){ :|:& };:

7.  Run this script as root (i.e. sudo ./forkbomb.sh) and wait.  This
    should ultimately cause the system to hang and result in a watchdog
    timeout and subsequent reboot.