[$] The trouble with iowait

CPU scheduling is a challenging job; since it inherently requires making
guesses about what the demands on the system will be in the future, it
remains reliant on heuristics, despite ongoing efforts to remove them.
Some of those heuristics take special note of tasks that are (or appear to
be) waiting for fast I/O operations.  There is some unhappiness, though,
with how this factor is used, leading to a couple of patches taking rather
different approaches to improve the situation.

https://lwn.net/Articles/989272/