[$] Revisiting the kernel's preemption model, part 2

In last week's episode, a need to preempt
kernel code that is executing long-running instructions led to a deeper
reexamination of how the kernel handles preemption. There are a number of
supported preemption modes, varying from 'none' (kernel code is never
preemptible) to realtime (where the kernel is almost always preemptible).
Making better use of the kernel's preemption machinery looked like a
possible solution to the immediate problem, but it seems that there are
better options in store. In short, kernel developers would like to give
the scheduler complete control over CPU-scheduling decisions.