Long and background tasks

What we miss in the previous approach is
Support for tasks that must use all "free" CPU time
Support for the random "long" job duration

Typical "long" tasks:
A command shell on the serial port
Data communication, over serial or USB (or whatever)
To support that, we really need preemption

We can special-case some of them
We accept that the console is just a debugging tool
It can temporary halt scheduling
And we know for sure it won't run in production

But data communication takes time
We must prepare our frames, possibly with printf too
This happens rarely, but then it's a few hundred microseconds
Or even several milliseconds on the UART port