[$] Timer IDs, CRIU, and ABI challenges

The kernel project has usually been willing to make fundamental internal
changes if they lead to a better kernel in the end. The project also,
though, goes out of its way to avoid breaking interfaces that have been
exposed to user space, even if programs come to rely on behavior that was
never documented. Sometimes, those two principles come into conflict,
leading to a situation where fixing problems within the kernel is either
difficult or impossible. This sort of situation has been impeding
performance improvements in the kernel's POSIX timers implementation for
some time, but it appears that a solution has been found.