Giovanni Gherdovich The four kernel preemption flavors in Linux

When Linus Torvalds announced Linux 0.01 on comp.os.minix in 1991 his "hobby, nothing big or professional" operating system had
preemptive multitasking already. But it wasn't a preemptive kernel: user mode code could be preempted, but kernel code still needed to
run until completion. Today Linux supports four different flavors of kernel preemption; this talk is a tutorial at what they do and how they
differ. Here is their list in chronological order of implementation, together with the associated build-time config option:
no kernel preemption; CONFIG_PREEMPT_NONE (Sep 1991, Linux 0.01, Linus Torvalds)
full kernel preemption; CONFIG_PREEMPT (Feb 2002, Linux 2.5.4, Robert Love)
real-time patch, known as "PREEMPT_RT" (Mar 2005, initial release based on Linux 2.6.11, Ingo Molnár and Thomas Gleixner)
voluntary kernel preemption; CONFIG_PREEMPT_VOLUNTARY (Aug 2005, Linux 2.6.13, Ingo Molnár)
Since Linux v5.12 (Apr 2021) we have the boot-time parameter "preempt="; it can be set to either "none", "voluntary" or "full" so this choice
doesn't need to be made at compile time anymore (except for PREEMPT_RT, which is still a separate kernel).