Sorry for my disturbance... I found there is a description table on gentoo's website. http://www.gentoo.org/dyn/use-index.xml It seems that I use the build flag when I was building the kernel, and forget to remove it from make.conf.
I could not found the module _elementtree in /usr/lib64/python2.7/lib-dynload/
I also try to rebuild it, the issue is still there...
Should I enable specific USE flag for python to build _elementtree module? Please help...