How can I get gpu utilization?

Question

Previously, I tried NVML by using the function nvmlDeviceGetUtilizationRates(). I test it by this way, while the collection is running, I excute a DFT ( the Kernel is organised as <7,32>) on Tesla C2070 which have 14 SMs, on my thought, there were 7 blocks excuted on GPU at the same time, and the utilization should be 50%, but the API gave the result as 99%, which means GPU be used completely. Then, I read the documents of NVML, the nvmlDeviceGetUtilizationRates() can only return the pecent of time that over the past sample period during which one or more kernels was executing on the GPU. How can I get the active SM numbers while some kernel is running on GPU?