[...]
What Meta is doing can indeed only reasonably be done with a sampling
profiler, with additional restrictions (in particular, they state that
Perf's support to drop back to userspace for DWARF unwinding is too slow for
them) [...]
(Note FWIW that systemtap's kernel runtime does DWARF-based unwinding
for the kernel and user-designated userspace executables and
shared-libraries on demand, and it's not particularly slow at it.)
- FChE