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# Tested on RHEL 5, 6 & 7
# What is the "Magic" 'SysRq' key? # ------------------------------------------------------------------------------------------ # According to the Linux kernel documentation: # # It is a 'magical' key combo you can hit which the kernel will respond to regardless of # whatever else it is doing, even if the console is unresponsive. # # The 'SysRq' key is one of the best (and sometimes the only) way to determine what a # machine is really doing. It is useful when a system appears to be "hung" or for # diagnosing elusive, transient, kernel-related problems. # How do I enable and disable the 'SysRq' key? # ------------------------------------------------------------------------------------------ # For security reasons, Red Hat Enterprise Linux disables the 'SysRq' key by default. To # enable it, run: echo 1 > /proc/sys/kernel/sysrq # Or: sysctl -w kernel.sysrq=1 # 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 description): # 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 # To disable it: echo 0 > /proc/sys/kernel/sysrq # Or: sysctl -w kernel.sysrq=0 # To enable it permanently, set the kernel.sysrq value to 1. That will cause it to be # enabled on start up # RHEL 5 & 6 vi /etc/sysctl.conf kernel.sysrq = 1 # RHEL 7 vi /usr/lib/sysctl.d/50-default.conf kernel.sysrq = 1 # Since enabling 'SysRq' gives you physical console access extra abilities, it is recommended # to disable it when not troubleshooting a problem or to ensure that physical console # access is properly secured. # How do I trigger a 'SysRq' event? # ------------------------------------------------------------------------------------------ # There are several ways to trigger a 'SysRq' event. On a normal system, with an AT # keyboard, events can be triggered from the console with the following key combination: # Alt+PrintScreen+[CommandKey] # For instance, to tell the kernel to dump memory info (command key "m"), you would hold # down the "Alt" and "Print Screen keys", and then hit the m key. # Note that this will not work from an X Window System screen. You should first change to # a text virtual terminal. Hit Ctrl+Alt+F1 to switch to the first virtual console prior to # hitting the 'SysRq' key combination. # On a serial console, you can achieve the same effect by sending a Break signal to the # console and then hitting the command key within 5 seconds. This also works for virtual # serial console access through an out-of-band service processor or remote console like # HP iLO, Sun ILOM and IBM RSA. # Refer to service processor specific documentation for details on how to send a Break # signal; for example, How to trigger SysRq over an HP iLo Virtual Serial Port (VSP). # If you have a root shell on the machine (and the system is responding enough for you to # do so), you can also write the command key character to the /proc/sysrq-trigger file. # This is useful for triggering this info when you are not on the system console or for # triggering it from scripts. echo 'm' > /proc/sysrq-trigger # When I trigger a 'SysRq' event that generates output, where does it go? # ------------------------------------------------------------------------------------------ # When a 'SysRq' command is triggered, the kernel will print out the information to the # kernel ring buffer and to the system console. This information is normally logged via # syslog to /var/log/messages. # Unfortunately, when dealing with machines that are extremely unresponsive, syslogd is # often unable to log these events. In these situations, provisioning a serial console is # often recommended for collecting the data. # What sort of 'SysRq' events can be triggered? # ------------------------------------------------------------------------------------------ # There are several 'SysRq' events that can be triggered once the 'SysRq' facility is # enabled. These vary somewhat between kernel versions, but there are a few that are # commonly used: # m - dump information about memory allocation # t - dump thread state information # p - dump current CPU registers and flags # c - intentionally crash the system (useful for forcing a disk or netdump) # s - immediately sync all mounted filesystems # u - immediately remount all filesystems read-only # b - immediately reboot the machine # o - immediately power off the machine (if configured and supported) # f - start the Out Of Memory Killer (OOM) # w - dumps tasks that are in uninterruptible (blocked) state |