指令如下：
su root
yum -y install https://www.elrepo.org/elrepo-release-7.el7.elrepo.noarch.rpm
rpm –import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
yum –enablerepo=elrepo-kernel install kernel-lt kernel-lt-devel
yum install nano -y
nano /etc/default/grub
更改：GRUB_DEFAULT=0，Ctrl+O回车保存，Ctrl+X退出
grub2-mkconfig -o /boot/grub2/grub.cfg
reboot