How to know the GPU usage and who is using it?

nvidia-smi --query-compute-apps=gpu_bus_id,pid,process_name,used_memory --format=csv,noheader,nounits

To know who or which user is using that process pid:-
ps -o pid,user,%cpu,%mem,etime,command -p "pid"
ps -o pid,user,%cpu,%mem,etime,command -p 3388