[root@yunhe ~]# service mysql stop
MySQL server PID file could not be found!????????????????? [失败]

解决方案如下：

首先查看下与mysql有关的进程

[root@yunhe ~]# ps aux |grep mysq*
root??????2019? 0.0? 0.1?? 6228? 1392 ???????? S??? 09:22?? 0:00 /bin/sh /usr/bin/mysqld_safe --datadir=/var/lib/mysql --pid-file=/var/lib/mysql/yunhe.pid
mysql???? 2115??0.0? 2.9 353596 30808 ???????? Sl?? 09:22?? 0:01 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib/mysql/plugin --user=mysql --log-error=/var/lib/mysql/yunhe.err --pid-file=/var/lib/mysql/yunhe.pid
500?????? 4258? 0.0? 1.6? 83432 17484 ???????? S??? 09:35?? 0:01 gedit /usr/share/mysql/my-medium.cnf
root????? 5555? 0.0? 0.0?? 5936?? 768 pts/0??? S+?? 10:04?? 0:00 grep mysq*

如果看到上面的内容，那说明mysql相关的进程存在，不过资源调度出现了问题（好像卡死），这时用就要把这些卡死的进程都关闭。
[root@yunhe ~]# kill 2019
[root@yunhe ~]# kill 20115
-bash: kill: (20115) - 没有那个进程
[root@yunhe ~]# kill 2115

然后重新启动mysql
[root@yunhe ~]# service mysql start
Starting MySQL...????????????????????????????????????????? [确定]
[root@yunhe ~]# mysql -u root -p
Enter password:?
Welcome to the MySQL monitor.? Commands end with ; or \g.
Your MySQL connection id is 1
Server version: 5.6.4-m7 MySQL Community Server (GPL)

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mysql>?