I have two, one is actually complicated and one was so obtuse that I never would have figured it out in a million years:

Actually complicated: I still don’t know how it happened, but somehow an update on Arch filled the boot partition with junk files, which then caused the kernel update to fail because of no disk space, which then kind of tanked the whole system. It took ages, but with a boot disk and chroot-ing back into the boot partition I eventually managed to untangle it all. I was determined to see it through and not reinstall.

Ridiculous: One day when using Ubuntu, the entire system went upside-down. As in, everything was working perfectly fine, but literally the screen was upside-down. After much Googling I had no luck figuring it out, then I accidentally found the solution - I’d plugged a PS4 controller into the USB on the laptop to charge it, and for some reason Ubuntu interpreted the gyroscope on the controller as “rotate the screen display” so when I moved it, the screen spun round. I only figured it out by accident when I plugged it back it and it spun back to normal lol.

Not fixed but there is an Arch problem that is and will always be the bane of mi existence.

For some reason when I click with the trackpad buttons the touchpad gets frozen for like a second (it’s like they are recognised by the system as keyboard buttons, I have enabled that option to temporarily disable it when using keyboard).

I’ve checked for hours and days the libinput documentation and some synaptics libraries, even legacy ones. It is to this day the only problem that has lead me to reinstall my system but the problem remains.

It’s not even like I have some niche setup, I mean, surely there must be thousands of Arch users running with a ThinkPad X1 Carbon Gen 7, and surely not every single one of them must be running it like this, right?

It has come to a point where I just gave up and got used to my system as is, but I’m sure I would be running fanfare if some day I am able to fix it.

I once exited vim without having to look up the commands.

I’ve found that the silliest desktop problems are usually the hardest to solve, and the “serious” linux system errors are the easiest.

System doesn’t boot? Look at error message, boot from a rescue disk, mount root filesystem and fix what you did wrong.

Wrong mouse cursor theme in some Plasma applications, ignoring your settings? Some weird font rendering issue? Bang your head against a wall exploring various dotfiles and rc files in your home directory for two weeks, and eventually give up and nuke your profile and reconfigure your whole desktop from scratch.

Grub.

Seriously. Tha was some fat as shit because I didn’t know what I was doing.

I manage a machine that runs both media transcodes and some video game servers.

The video game servers have to run in real-time, or very close to it. Otherwise players using them suffer noticeable lag.

Achieving this at the same time that an ffmpeg process was running was completely impossible. No matter what I did to limit ffmpegs use of CPU time. Even when running it at lowest priority it impacted the game server processes running at top priority. Even if I limited it to one thread, it was affecting things.

I couldn’t understand the problem. There was enough CPU time to go around to do both things, and the transcode wasn’t even time sensitive, while the game server was, so why couldn’t the Linux kernel just figure it out and schedule things in a way that made sense?

So, for the first time I read up on how computers actually handle processes, multi-tasking and CPU scheduling.

As FFMPEG is an application that uses ALL available CPU time until a task is done, I came to the conclusion that due to how context switching works (CPU cores can only do one thing, they just switch out what they do really fast, but this too takes time) it was causing the system to fall behind on the video game processes when the system was operating with zero processing headroom. The scheduler wasn’t smart enough to maintain a real-time process in the face of FFMPEG, which would occupy ALL available cycles.

I learned the solution was core pinning. Manually setting processes to run on certain cores of the CPU. I set FFMPEG to use only one core, since it doesn’t matter how fast it completes. And I set the game processes to use all but that one core, so they don’t accidentally end up queueing for CPU time on a core that doesn’t have the headroom to allow the task to run within a reasonable time range.

This has completely solved the problem, as the game processes and FFMPEG no longer wait for CPU cycles in the same queue.