Note: This article is currently only a draft and is a part of a

series of articles I’m going to publish the next few months – if you

want to contribute to it, please feel free to. However it would be nice

if you could coordinate your efforts with

me

This article is still work in progress and still needs a lot of effort.

Contents

Introduction

MagicSysRq is a special key combination which lets the user perform

certain low level tasks of the kernel and is especially useful when the

whole system seems to be hung. MagicSysRq e.g. makes it possible to,

more or less, cleanly shutdown a system, which doesn’t even respond to

CTRL+ALT+DEL any more.

The MagicSysRq functions are invoked by pressing ALT+SysRq+Function Key

where function key describes one of the following.

TODO:Insert table here

Example

The probably most used combination of MagicSysRq functions is the one

RSEIUB, which can be easyly remebered by the mnemonic trick “Raising

Elephants is so utterly boringfootnote{“You can still find several

references the mnemonic Raising Skinny Elephants is utterly boring on

the internet – however it is probably better to do the sync after the

tasks exited/were killed} – although we’re dealing with penguins here 😉

According to the table above this performs, in order: R Put Keyboard in

Raw Mode E terminate all tasks I kill all tasks S Sync U remount all

filesystems read-only B Reboot

Troubleshooting

You can easily and safely check whether MagicSysRq works on your System

by pressing Alt+SysRq+H – on a console you should now see a short help

text of the MagicSysRq Feature or at least print this to `dmesg`.

If you don’t see the help text, check whether CONFIG_MAGIC_SYSRQ=y

is set in your Kernel Config. If it is set, please check the output of

‘cat /proc/sys/kernel/sysrq’ which should report 1 if MagicSysRq is

enabled. To enable it just do a simple echo 1 > /proc/sys/kernel/sysrq

Important Files

/proc/sys/kernel/sysrq – enable/disable MagicSysRq at runtime

/proc/sysrq-trigger – trigger MagicSysRq from the console (e.g. useful

over ssh)

TODO Content:

Explanation of remaining keys

using it over serial line -> BREAK+key, in kermit ctrl- b KEY

Interaction with KDB

Interaction with kexec / crashkernel

External links

總結

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