For those who find the 6.x kernel intimidating, Seiya Nuta has written a look at the 0.01
kernel, which reflects a simpler time.

By the way, there’s an interesting comment about the scheduler:

    *  'schedule()' is the scheduler function. This is GOOD CODE! There
    * probably won't be any reason to change this, as it should work well
    * in all circumstances (ie gives IO-bound processes good response etc).

Yes it’s indeed good code. Unfortunately (or fortunately), this
prophecy is false. Linux became one of most practical and
performant kernel which has introduced many new scheduling
improvements and algorithms over the years, like Completely Fair
Scheduler (CFS).

Source: LWN.net – Nuta: Exploring the internals of Linux v0.01