It is eternally frustrating how valuable kernel features like grsec, the bfq and bfs schedulers (for disk and cpu respectively), and many more hugely useful but not commercially backed (or at least proposed) projects wallow in side trees of Linux mainline. It is just baffling how the grsec patches are ~13? years old now and yet remain out of tree rather than, say, default-off on kconfig, which even then makes no sense - users of the kernel should be getting maximally secure and conservative software first, and then turning off checks to increase performance, not the other way around - having a wild and exploitable kernel you need to use out of tree patches to get under reasonable control.