查看系统可用内核：

cat /boot/grub2/grub.cfg |grep menuentry

...
menuentry 'CentOS Linux (4.17.8-1.el7.elrepo.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-4.17.8-1.el7.elrepo.x86_64-advanced-fe0109f2-6f34-48ae-b51e-1f5fa78305b5' {
menuentry 'CentOS Linux (3.10.0-862.9.1.el7.x86_64) 7 (Core)' --class centos --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-862.9.1.el7.x86_64-advanced-fe0109f2-6f34-48ae-b51e-1f5fa78305b5' {
...

查看当前内核：

uname -r

3.10.0-862.9.1.el7.x86_64

修改开机时默认使用的内核：

grub2-set-default 'CentOS Linux (4.17.8-1.el7.elrepo.x86_64) 7 (Core)'

查看内核修改结果：

grub2-editenv list

saved_entry=CentOS Linux (4.17.8-1.el7.elrepo.x86_64) 7 (Core)

查看系统安装了哪些内核包：

rpm -qa |grep kernel

kernel-tools-libs-3.10.0-862.9.1.el7.x86_64
kernel-ml-4.17.8-1.el7.elrepo.x86_64
kernel-tools-3.10.0-862.9.1.el7.x86_64
kernel-headers-3.10.0-862.9.1.el7.x86_64
kernel-devel-3.10.0-862.9.1.el7.x86_64
kernel-3.10.0-862.9.1.el7.x86_64

使用yum remove 或rpm -e 删除无用内核：

yum remove kernel-ml-3.10.0-862.9.1.el7.x86_64