Mysql using 100% cpu with no processes

Hi

I’ve noticed a high CPU usage for my mysql server running on ubuntu 16.04.

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
9646 mysql 20 0 2021452 299272 20756 S 102.3 0.5 18:01.34 mysqld

mysql> SHOW FULL PROCESSLIST;
±-----±-----±----------±-----±--------±-----±---------±----------------------+
| Id | User | Host | db | Command | Time | State | Info |
±-----±-----±----------±-----±--------±-----±---------±----------------------+
| 1431 | root | localhost | NULL | Query | 0 | starting | SHOW FULL PROCESSLIST |
±-----±-----±----------±-----±--------±-----±---------±----------------------+
1 row in set (0.00 sec)

Virtualmin reports a CPU load at about 50%, and process size = 1.99 GiB. If I restart the mysql server, then load is low in a few minutes, then it builds up again.

My mysql.cnf:

[mysqld]
sql_mode=IGNORE_SPACE,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION

query_cache_type=1
query_cache_limit=512K
query_cache_size=64M
query_cache_min_res_unit=256
key_buffer=64M
key_buffer_size=64M
innodb_buffer_pool_size=512M
innodb_thread_concurrency=0
innodb_log_file_size=128M

Can you give any advise on how to trace down the issue?