SCHEDULING OF DUPLICATE THREADS

A processing system includes safety thread scheduling circuitry which schedules a check thread, being a duplicate of a critical thread, to be executed on a second of a plurality of parallel execution units other than a first execution unit upon which the critical thread is run. The processor further includes comparison circuitry which will compare a result of the critical thread with a result of the check thread, and raise error signal if the results do not match. The safety thread scheduling circuitry is configured to detect when one of the execution units is idle, and if none of the execution units is detected to be idle by the expiry of a safety time-window, to interrupt a non-critical thread executing on a non-idle one of the execution units and select the non-idle execution unit as the second execution unit to execute the check thread in place of the interrupted thread.