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What is the magic SysRq key?

It is a 'magical' key combo you can hit which the kernel will respond to regardless of whatever else it is doing, unless it is completely locked up.

How do I enable the magic SysRq key?

You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when configuring the kernel. When running a kernel with SysRq compiled in, /proc/sys/kernel/sysrq controls the functions allowed to be invoked via the SysRq key. By default the file contains 1 which means that every possible SysRq request is allowed (in older versions SysRq was disabled by default, and you were required to specifically enable it at run-time but this is not the case any more). Here is the list of possible values in /proc/sys/kernel/sysrq:

  0 - disable sysrq completely
  1 - enable all functions of sysrq
 >1 - bitmask of allowed sysrq functions (see below for detailed function
         2 - enable control of console logging level
         4 - enable control of keyboard (SAK, unraw)
         8 - enable debugging dumps of processes etc.
        16 - enable sync command
        32 - enable remount read-only
        64 - enable signalling of processes (term, kill, oom-kill)
       128 - allow reboot/poweroff
       256 - allow nicing of all RT tasks

You can set the value in the file by the following command:

   echo "number" >/proc/sys/kernel/sysrq

Note that the value of /proc/sys/kernel/sysrq influences only the invocation via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always allowed (by a user with admin privileges).