Author here. Scheduling is a spectrum. Current OS are preemptive, but also a bit cooperative. An app can make the decision of yielding control.
The opposite could work: OS is cooperative, unless some threshold of resource usage is triggered (a timer interrupt of instance). It then context switch to enter a, hopefully rare, failure mode, thus turning preemptive. Kill the app, and get back into cooperative mode. Let's call it optimistically cooperative & pessimistically preemptive.
The opposite could work: OS is cooperative, unless some threshold of resource usage is triggered (a timer interrupt of instance). It then context switch to enter a, hopefully rare, failure mode, thus turning preemptive. Kill the app, and get back into cooperative mode. Let's call it optimistically cooperative & pessimistically preemptive.