You need to turn on RT_VFP_LAZY_STACKING in rtconfig.h. By default, RTT
will turn on VFP for all threads and stack all the VFP registers. When
doing lazy stacking, VFP will only be turned on for the thread who
issued VFP instructions.

Currently, if a thread turned on VFP, it cannot be turned off. RTT will
never know what time the thread doesn't need VFP any more. The thread
might could turn off the VFP for it self in proper time.