Preemptive scheduling enclaves as disclosed herein support both cooperative and preemptive scheduling of in-enclave (IE) thread execution. These preemptive scheduling enclaves may include a scheduler configured to be executed as part of normal hardware interrupt processing by enclave threads. The scheduler identifies an IE thread to be scheduled and modifies enclave data structures so that when the enclave thread resumes processing after a hardware interrupt, the identified IE thread is executed, rather than the interrupted IE thread.