Does anyone have more reading on this? Sounds interesting. I know Windows 7+ x64 has a user-mode scheduler but I hadn't heard of a Linux-based OS supporting this. How does it handle blocking calls and such?

they're not a userspace construct in any meaningful way. libutils/Thread (capital-T Thread) is a thin wrapper around pthread_t, and most Android code uses pthread_t directly. what this hit is probably a bug related to timerslack_ns--there's some code to tweak timerslack values on a per-thread basis instead of a per-process basis even though audio should never be coming from processes low enough on the importance list to get a high timerslack value--but L bugs fixed in M are before my time.