Reading : https://www.tensorflow.org/versions/r0.10/resources/faq.html it states :
Does TensorFlow make use of all the devices (GPUs and CPUs) avail
CPUs are used via a "device" which is just a threadpool. You can control the number of threads if you feel like you need more:
sess = tf.Session(config=tf.ConfigProto( intra_op_parallelism_threads=NUM_THREADS))