Buried in the Wikipedia article is what I remember, with a note saying not to use it any more.
So what sysreq keys do you use to recover a non responsive system with a journaling file system?
Before the advent of journaled filesystems a common use of the magic SysRq key was to perform a safe reboot of a locked-up Linux computer (using the sequence of key presses indicated by the mnemonic REISUB), which lessened the risk of filesystem corruption. With modern filesystems, syncing and unmounting is still useful to force unflushed data to disk, but is no longer necessary to prevent filesystem corruption (and may increase the risk of corruption in case the lock-up is caused by the kernel being in a bad state).[11] The default value of kernel.sysrq in distributions such as Ubuntu and Debian remains 176[1]^ (allowing the sync, unmount, and reboot functions) and 438[12] (allowing the same functions plus loglevel, unraw, and nice-all-RT-tasks) respectively.
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In case anyone needed a better explanation of what REISUB stands for:
R.E.I.S.U.B.
Raising Elephants Is So Utterly Boring
R.E.I.S.U.B. is a key sequence used on Linux systems to safely reboot a frozen or unresponsive computer. It is often referred to as the “magic SysRq key” (System Request key) and allows users to perform low-level commands to safely recover or restart the system.
The acronym R.E.I.S.U.B. stands for the specific sequence of commands you can execute to reboot the system safely, even if the system is otherwise unresponsive:
R - Raw mode (take control of the keyboard back from the X server). E - End all running processes cleanly (sends SIGTERM to all processes). I - Immediately kill all processes (sends SIGKILL to all processes). S - Sync the disk (flushes data to disk to prevent data loss). U - Unmount all file systems (remounts all filesystems as read-only). B - Boot (reboots the system).
How to Use R.E.I.S.U.B. Safely
Hold Alt and SysRq, and then press each letter separately in the order of R-E-I-S-U-B. However, often the SysRq key is a dual purpose key which needs another key to activate the secondary function. For example, on a Acer keyboard SysRq is a secondary function of the Delete key and the Fn key is used to select the secondary function. So on this keyboard it would require holding down the Alt + Fn + Delete to activate the SysRq function.
From http://pclosusers.com/wiki/index.php?title=REISUB_reboot_your_system>
I’ve always translated R.E.I.S.U.B to Reboot Even If System Utterly Broken
It doesn’t always work though, if it’s linux itself crashing you’re out of luck.
The clue is in the directions: REISUB
- Hold Alt + Print Scr
- Then type R E I S U B slowly.
- Give a 2-3sec pause between presses to be sure.
You can look up what each letter does, as each one performs a different function when combined with Alt + Print Scr.
ETA: I seem to recall that you can enter the sequence quicker than that, as I believe I used to, but it’s probably good to give your system time to perform each function, especially if you’re trying to rescue it.
For me all of those key presses produce dmesg logs that just state all of those are disabled
The function I used most often aside from the usual sequence is Alt-SysRq-F. This runs
oomkillerto SIGKILL what would appear to be the process hogging the most memory. That was usually enough to get control back from an agonizingly sluggish system caused by hitting the swapspace. I didn’t need it anymore after I upgraded to 48GB of RAM.Another way to remember the classic mnemonic, it’s “busier” in reverse, after holding down for a second the Alt and SysRq keys (which may require a function key combo on some keyboards).
You should check ahead of time whether the value of
/proc/sys/kernel/sysrqis not zero to be sure the Sysrq functions are enabled. If not, then in/etc/sysctl.d/99-sysctl.confyou can add the linekernel.sysrq = 1to enable all features, or a higher number to enable select features.Thanks, I’ll try this next time
If you have a Lenovo laptop, these are the keys:
Press Alt key (or whatever key maps to Alt, if you’ve made key remappings) and keep it pressed all the time, while doing as follows, where “Fn” is the function key:
- Press Fn+s, release Fn+s, press r
- Press Fn+s, release, press e
- Press Fn+s, release, press i
- Press Fn+s, release, press s
- Press Fn+s, release, press u
- Press Fn+s, release, press b
You see the sequence gives “REISUB”.
When pressing keys or key combos, keep them pressed for a second or so. If you press a wrong key, restart from the beginning.
Good luck! 🍀💪
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No magic. It’s all just software.
I did
REJa few times to recover from problems with a VM causing problems with S3 sleep.