Functions that do not create a stack frame on x86 systems cannot be instrumented by FBT. Because the register set for x86 is extraordinarily small, most functions must put data on the stack and therefore create a stack frame. However, some x86 functions do not create a stack frame and therefore cannot be instrumented. Actual numbers vary, but typically fewer than five percent of functions cannot be instrumented on the x86 platform.
Leaf routines hand-coded in assembly language on SPARC systems cannot be instrumented by FBT. The majority of the kernel is written in C, and all functions written in C can be instrumented by FBT.