正是申请季高峰，但还是被老师抓着干活。之前一直以为cuda已经装好，才知道是骗自己的。我的显卡是640，ubuntu14.4， 比较low。
我是按照这个教程走的，http://m.blog.csdn.net/article/details?id=51882183，当走到sudo sh cuda_7.5.18_linux.run这步时，出现了一个bug:
An NVIDIA kernel module 'nvidia' appears to already be loaded in your
kernel. This may be because it is in use (for example, by the X
server), but may also happen if your kernel was configured without
support for module unloading. Please be sure you have exited X
before attempting to upgrade your driver. If you have exited X, know
that your kernel supports module unloading, and still receive this
message, then an error may have occured that has corrupted the NVIDIA
kernel module's usage count; the simplest remedy is to reboot your
computer.
当时就懵了，sudo service lightdm stop 肯定能把X Server关闭了，那第二个可能性就可能是罪魁祸首啦，an error may have occured that has corrupted the NVIDIA kernel module's usage count。
大概意思就是跟NVIDIA有关，要删除以前旧的驱动，然后装新驱动，其实我也是半知半解，但我找到了一个帖子解决了 我的问题 http://askubuntu.com/questions/830916/how-to-install-cuda-8-0-on-ubuntu-16-04-with-nvidia-geforce-gtx-1080
关键的命令：
sudo apt-get purge nvidia-cuda*
sudo apt-get purge nvidia-*
详情参考帖子原文。
由于时间很紧，写的比较乱，由于每个人的配置不尽相同，能用这个方法解决当然最好，不行的话，也不能勉强。