Gfilui

My (Ubuntu 16.04) laptop has the following quirk: usually shutdown will take under 5 seconds, occasionally it will take over a minute. Never anything in between.

Having observed this phenomenon for almost half a year now, the only thing I am sure of is this: the long shutdowns happen exclusively if - since booting up - the WiFi has at some point been enabled. They do not always happen if WiFi has been enabled, but it seems to be a necessary condition. On the other hand, disabling the WiFi in the network manager GUI right before shutdown is not sufficient to prevent the minute-long delay.

Yet, looking at the logging during shutdown, no mention of anything WiFi-related is made. During such long shutdowns, I immediately get

[ OK ] Started Show Plymouth Power Off Screen
[ OK ] Stopped Thermal Deamon Service

Then it stalls for one or two minutes, then the rest of the shutdown procedure happens in less than a second, starting with

[ OK ] Stopped session c2 of user *username*
[ OK ] Removed slice User slice of *username*

and ending with

[ OK ] Stopped Remount Root and Kernel Systems
[ OK ] Reached Target Shutdown

Nowhere is there any mention of the WiFi hardware or any error message. CUPS is somewhere in the middle of these logs; I know that it is often the culprit in slow-shutdown situations, but I do not have any printers on the network.

Let me know if you need more details on the shutdown logs, I recorded it on camera and did not feel like retyping all of it.