一个watchdog deamon的实例

Problem

A remote machine that is hard to reach gets stuck, e.g. because all memory has been consumed by some rogue process. We want it to reboot automatically.

Solution

• We want the kernel to reboot upon a panic. man 7 bootparam tells us that this can be achieved by giving the parameter panic=N (to be put in the ``kernel'' line in /boot/grub/menu.lst (on gentoo). This ensures that the kernel will reboot (after N secs) upon panic.

  The same effect can be achieved by

  echo N >/proc/sys/kernel/panic

  (I'm not sure whether that is really needed for the watchdog daemon, but it won't hurt either).

• Turn on support for the software /dev/watchdog device in the kernel. This can be done in make menuconfig: select character devices (``watchdog cards''), turn on the main option and software watchdog (the ``softdog'' driver). I don't want to bother with modules so I just added the driver to the kernel (after testing as modules).

  Note that activating the softdog driver does not force you to have a watchdog daemon to ``pet the dog''. For that, the /dev/watchdog file must be opened first.

  There are 2 possible parameters for the softdog driver as appears from the code snippet below:

  	#define TIMER_MARGIN	60		/* (secs) Default is 1 minute */
  
  	static int soft_margin = TIMER_MARGIN;	/* in seconds */
  	#ifdef ONLY_TESTING
  	static int soft_noboot = 1;
  	#else
  	static int soft_noboot = 0;
  	#endif  /* ONLY_TESTING */
  
  	MODULE_PARM(soft_margin,"i");
  	MODULE_PARM(soft_noboot,"i");
      

  Thus, without any parameters, the machine will halt if the file /dev/watchdog, when open, has not been written to in 60 secs.
• Install the watchdog daemon package (available ). I use gentoo, so emerge watchdog does the lot. If the device /dev/watchdog does not appear automatically, you can create it using

    mknod /dev/watchdog c 10 130
    

  Since I'd like to know what was going on when the machine got stuck, I specified a user-defined test program /usr/sbin/watchdog-user-test in /etc/watchdog/watchdog.conf which is reproduced below.

    #ping			= 172.31.14.1
    #ping			= 172.26.1.255
    #interface		= eth0
    #file			= /var/log/messages
    #change			= 1407
  
    # Uncomment to enable test. Setting one of these values to '0' disables it.
    # These values will hopefully never reboot your machine during normal use
    # (if your machine is really hung, the loadavg will go much higher than 25)
    #max-load-1		= 24
    #max-load-5		= 18
    #max-load-15		= 12
  
    # Note that this is the number of pages!
    # To get the real size, check how large the pagesize is on your machine.
    #min-memory		= 1
  
    repair-binary		= /usr/sbin/repair
    test-binary		= /usr/sbin/watchdog-user-test
  
  
    #watchdog-device	= /dev/watchdog
    
    # Defaults compiled into the binary
    #temperature-device	=
    #max-temperature	= 120
    
    # Defaults compiled into the binary
    #admin			= root
    #interval		= 10
    #logtick                = 1
    
    # This greatly decreases the chance that watchdog won't be scheduled before
    # your machine is really loaded
    realtime		= yes
    priority		= 1
  
    # Check if syslogd is still running by enabling the following line
    #pidfile		= /var/run/syslogd.pid   
    

  Here is /usr/sbin/watchdog-user-test:

    #!/bin/sh
    # 
    { date; ps -efl; } >/var/log/watchdog-user-test-output
    

  Finally, to have the watchdog daemon start at boot time, it suffices (in gentoo) to do rc-update add watchdog default.

References

• : how to upgrade openssh without rebooting
• daemon sources and further info

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