howto/upgrades.md

@@ -306,4 +306,30 @@ See the [Ganeti reboot procedures](howto/ganeti#rebooting) for this procedure.
 ## Remaining nodes
 
 The [Nagios unhandled problems](https://nagios.torproject.org/cgi-bin/icinga/status.cgi?allunhandledproblems) will show remaining hosts that
-might have been missed by the above procedure..
+might have been missed by the above procedure.
+
+## Userland reboots
+
+systemd 254 (Debian 13 trixie and above) has a special command:
+
+    systemctl soft-reboot
+
+That will "shut down and reboot userspace". As the [manual page
+explains](https://manpages.debian.org/testing/systemd/systemd-soft-reboot.service.8.en.html):
+
+> systemd-soft-reboot.service is a system service that is pulled in by
+> soft-reboot.target and is responsible for performing a
+> userspace-only reboot operation. When invoked, it will send the
+> SIGTERM signal to any processes left running (but does not follow up
+> with SIGKILL, and does not wait for the processes to exit). If the
+> /run/nextroot/ directory exists (which may be a regular directory, a
+> directory mount point or a symlink to either) then it will switch
+> the file system root to it. It then reexecutes the service manager
+> off the (possibly now new) root file system, which will enqueue a
+> new boot transaction as in a normal reboot.
+
+This can therefore be used to fix conditions where systemd itself
+needs to be restarted, or a lot of processes need to, but not the
+kernel.
+
+This has not been tested, but could speed up some restart conditions.