Enable REISUB on Arch Linux

If you’ve used Linux long enough, you’ve hit a hard freeze. Mouse stuck, keyboard dead, Ctrl+Alt+F2 does nothing. Usually NVIDIA’s fault. Sometimes GNOME on Wayland. Sometimes just an unlucky kernel.

Your instinct is to hold the power button. Don’t. The kernel has a safer escape hatch baked in: Magic SysRq, and the REISUB sequence.

The catch on Arch: it’s disabled by default. Here’s how to fix that.

What is REISUB?

REISUB is a mnemonic for six Magic SysRq key presses. You hold Alt + SysRq (the Print Screen key on most keyboards) and tap each letter in order:

Key	Stands for	What it does
R	unRaw	Take keyboard control back from X/Wayland
E	tErminate	Send `SIGTERM` to every process (graceful)
I	kIll	Send `SIGKILL` to every process (forceful)
S	Sync	Flush all data to disk
U	Unmount	Remount all filesystems read-only
B	reBoot	Reboot immediately

The point is that all of this happens inside the kernel, completely below your desktop environment. So when GNOME Shell, Mutter, the NVIDIA driver, X/Wayland, or whatever other cursed piece of shit has your PC tied up and useless, the kernel can still hear you — unlike Winshit or Macrash.

When you actually need this

Real-world freezes I’ve watched people hit:

NVIDIA driver hangs. nvidia-drm deadlocks after suspend/resume, or after a Vulkan/CUDA workload faceplants. Screen frozen, GPU unresponsive.
GNOME / Mutter wedges. Especially on Wayland with mixed-refresh-rate monitors, fractional scaling, or that one extension you forgot you installed.
OOM storms. The OOM killer is famously not in a hurry. The machine thrashes swap for minutes before coming back.
Filesystem stalls. A hung NFS mount, a dying SSD, or a USB drive that timed out can lock anything touching I/O.
Kernel regressions. It happens. Especially on a rolling release.

REISUB gets you out of any of these cleanly. The alternative is the power button, which earns you:

Filesystem corruption (ext4 and btrfs both have opinions about being cut off mid-write).
Lost work in unsaved buffers.
A long fsck on next boot, or a journal replay that doesn’t quite recover.

Why it’s off by default on Arch

Two things are working against you.

First, the kernel itself. Arch ships its kernel with CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE=0x0, so SysRq is off the moment it boots.

Second, userspace. /usr/lib/sysctl.d/50-default.conf (shipped by systemd) sets:

kernel.sysrq = 16

That 16 is a bitmask, and it enables only S (Sync). The S in REISUB technically works. The R, E, I, U, and B do not. So on a fresh Arch install, REISUB does nothing you’d notice.

The bitmask, briefly

kernel.sysrq is 0 (off), 1 (everything on), or a sum of these bits:

Bit	Meaning
`2`	Console log level control
`4`	Keyboard control (un-raw, SAK)
`8`	Process dumps
`16`	Sync
`32`	Remount read-only
`64`	Signal processes (term, kill, OOM)
`128`	Reboot / poweroff
`256`	Renice RT tasks

Add what you want, or just use 1.

Is enabling it a security risk?

Kind of, but probably not for you.

What someone with physical keyboard access gains from kernel.sysrq = 1:

Reboot or power off the machine instantly.
Kill all your processes, which works around the lock screen if their goal is to ruin your session.
Trigger a kernel crash dump or summon the OOM killer.

What they don’t gain:

No code execution.
No privilege escalation.
No file reads or data exfiltration.
No way past disk encryption. LUKS still asks for your passphrase on the next boot.

Realistic risk on a personal laptop: basically nothing. Anyone with physical access can already hold the power button, yank the plug, or boot from a USB stick. REISUB hands them nothing new.

For a single-user Arch box, 1 is the standard recommendation.

How to enable it

Drop a file into /etc/sysctl.d/ to override the systemd default.

sudo tee /etc/sysctl.d/99-sysrq.conf <<'EOF'
# Enable all Magic SysRq functions (REISUB available during freezes)
kernel.sysrq = 1
EOF

Apply it without rebooting:

sudo sysctl --system

Verify:

cat /proc/sys/kernel/sysrq
# 1

Done. It survives reboots and package updates because of how /etc/sysctl.d/ precedence works.

Trying it out

You can test that SysRq is wired up without rebooting anything:

echo t | sudo tee /proc/sysrq-trigger

Then check dmesg. You should see a dump of every task’s state. If the output’s there, it’s working.

To rehearse the actual key combo, the safe letter is H (“help”). It just prints the list of available commands to the kernel log:

dmesg | tail

How to actually use REISUB in a freeze

When the system locks up:

Hold Alt and SysRq (PrtSc). Don’t let go.
Tap each letter in order, about a second apart:
```
R → E → I → S → U → B
```
The machine reboots cleanly.

Mnemonic: “Reboot Even If System Utterly Broken.” Or, backwards, “BUSIER.” Pick whichever sticks.

On laptops without a SysRq key

A lot of laptops bury SysRq behind Fn. So the combo becomes Fn + Alt + PrtSc while you tap each letter. Test with H first, so you know the exact chord on your specific keyboard before you need it in an emergency.

TL;DR — Arch ships Magic SysRq mostly disabled (kernel.sysrq = 16, sync only). One line in /etc/sysctl.d/ flips it to 1, and REISUB becomes your way out the next time NVIDIA, GNOME, or the kernel decides today’s the day.