报错提示如下：

http://mirrors.tencentyun.com/centos/%24releasever/extras/x86_64/repodata/repomd.xml: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
To address this issue please refer to the below knowledge base article

https://access.redhat.com/articles/1320623

If above article doesn't help to resolve this issue please create a bug on https://bugs.centos.org/

分析

yum源中$releasever不能被系统识别 - 简书 (jianshu.com)

http://mirrors.tencentyun.com/centos/%24releasever/extras/x86_64/repodata/repomd.xml 中的 %24releasever 没有解析出来导致。

yum中的变量$releasever是由/etc/yum.conf中的distroverpkg定义。在yum.conf中，显示为

distroverpkg=centos-release

centos-release 是一个rpm包。

经过检查，故障服务器没有安装 centos-release 的包。正常如下：

[root@VM_10_11_centos yum.repos.d]# rpm -qa |grep release
centos-release-7-4.1708.el7.centos.x86_64

解决

centos-release-7-9.2009.0.el7.centos.x86_64.rpm CentOS 7 Download (pkgs.org)

wget http://mirror.centos.org/centos/7/os/x86_64/Packages/centos-release-7-9.2009.0.el7.centos.x86_64.rpm
rpm -ivh centos-release-7-9.2009.0.el7.centos.x86_64.rpm

再尝试 yum ，成功

[root@VM_10_16_centos yum.repos.d]# yum repolist
Loaded plugins: fastestmirror, langpacks
epel                                                                                                                                                                                                                                                   | 4.7 kB  00:00:00     
(1/3): epel/7/x86_64/group_gz                                                                                                                                                                                                                          |  96 kB  00:00:00     
(2/3): epel/7/x86_64/updateinfo                                                                                                                                                                                                                        | 1.0 MB  00:00:00     
(3/3): epel/7/x86_64/primary_db                                                                                                                                                                                                                        | 7.0 MB  00:00:01     
Loading mirror speeds from cached hostfile
repo id                                                                                                                    repo name                                                                                                                                    status
epel/7/x86_64                                                                                                              EPEL for redhat/centos 7 - x86_64                                                                                                            13,745
extras/7/x86_64                                                                                                            Qcloud centos extras - x86_64                                                                                                                   509
os/7/x86_64                                                                                                                Qcloud centos os - x86_64                                                                                                                    10,072
updates/7/x86_64                                                                                                           Qcloud centos updates - x86_64                                                                                                                3,570
repolist: 27,896