From time to time, often after I've restored from sleep or finished playing a Steam game, one of my CPU cores is pinned at 100% with no indication of what might be doing it. Running htop, btop, or GNOME system monitor all show the same thing: CPU0 at 100% while the rest are doing near-nothing, and no process in particular seems to be using those resources.
If I restart, it's back to normal, and sometimes I can play a game in Steam or let the computer go to sleep and it doesn't do this, but it happens often enough that's annoying/confusing so I'd like to know if there's a way to either (a) diagnose which processes are using which CPU cores, or (b) somehow "reset" the checking of these values to make sure that something's not just being misreported.
This is a desktop system running Arch & GNOME.
In one of the other comments, we worked out that it was definitely something to do with ACPI, but yes I do have an external monitor. This is a desktop system.
Disabling the interrupt did the job, but I don't know why it's happening. If this is related to the monitor, could this be an Nvidia thing?
I have a pretty old integrated Intel GPU. Happens to my Thinkpad pretty regularly.