[$] Timer IDs, CRIU, and ABI challenges

The kernel project has usually been willing to make fundamental internalchanges 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 beenexposed to user space, even if programs come to rely on behavior that wasnever documented. Sometimes, those two principles come into conflict,leading to a situation where fixing problems within the kernel is eitherdifficult or impossible. This sort of situation has been impedingperformance improvements in the kernel's POSIX timers implementation forsome time, but it appears that a solution has been found.