$innobackupex --user=admin --password="xxxxxx" --socket=/u01/mysql/run/mysql.sock /data/backup/
xtrabackup: recognized server arguments: --datadir=/var/lib/mysql
xtrabackup: recognized client arguments:
211206 10:40:54 innobackupex: Starting the backup operation
IMPORTANT: Please check that the backup run completes successfully.
At the end of a successful backup run innobackupex
prints "completed OK!".
211206 10:40:54 version_check Connecting to MySQL server with DSN 'dbi:mysql:;mysql_read_default_group=xtrabackup;mysql_socket=/u01/mysql/run/mysql.sock' as 'admin' (using password: YES).
211206 10:40:54 version_check Connected to MySQL server
211206 10:40:54 version_check Executing a version check against the server...
211206 10:40:54 version_check Done.
211206 10:40:54 Connecting to MySQL server host: localhost, user: admin, password: set, port: not set, socket: /u01/mysql/run/mysql.sock
Using server version 5.7.25-log
Warning: option 'datadir' points to nonexistent directory '/var/lib/mysql'
Warning: option 'datadir' has different values:
'/var/lib/mysql' in defaults file
'/u01/mysql/data/' in SHOW VARIABLES
innobackupex version 2.4.15 based on MySQL server 5.7.19 Linux (x86_64) (revision id: 544842a)
xtrabackup: uses posix_fadvise().
innobackupex: Can't change dir to '/var/lib/mysql' (Errcode: 2 - No such file or directory)
xtrabackup: cannot my_setwd /var/lib/mysql

Solution:
MySQL instance for custom installation, modify mv /etc/my.cnf /etc/my.cnf.bk