RHEL kernels come with SysRq compiled in by default, but most distributions including Fedora disable this ‘Magic System Request’ functionality, at the boot time.
To see if it is already enabled:
$ cat /proc/sys/kernel/sysrq 0
0 means that the SysRq is disabled.
Here in my case on Stock Fedora 23 it returned the value 64 which means currently only the signalling functions are enabled.
To enable SysRq on your system, run:
$ echo "1" > /proc/sys/kernel/sysrq
To make the change persistent across reboots, edit
/etc/sysctl.d/99-sysrq.conf and add the following line:
kernel.sysrq = 1
Here are some of the magic key combos from recovering from a Fedora freeze:
While holding Alt and SysRq, type REIUSB
REIUSB basically kills the process, remount the filesystems read-only and will try to perform a soft (safe) reboot.
You can do REIUSO to do a shutdown instead.