The MariaDB 5.1.49 server wants read access to /sys/devices/system/cpu/ , but access to this directory is denied by its apparmor profile. This leads to entries in syslog, such as:

Aug 11 09:25:30 cachaca kernel: [1466836.273692] type=1503 audit(1281482730.279:83): operation="open" pid=1807 parent=1806 profile="/usr/sbin/mysqld" requested_mask="r::" denied_mask="r::" fsuid=0 ouid=0 name="/sys/devices/system/cpu/"

I first noticed this when the server started using 100% of one CPU core and I checked the logs in response. However, I've not been able to replicate that behaviour, so that may have been unrelated.

The problem can be resolved by adding the following line to /etc/apparmor.d/usr.sbin.mysqld:

/sys/devices/system/cpu/ r,