librenms/includes/polling/processors/ucd-old.inc.php

<?php

// Simple poller for UCD old style CPU. will always poll the same index.
// $system = snmp_get($device, "ssCpuSystem.0", "-OvQ", "UCD-SNMP-MIB");
// $user = snmp_get($device, "ssCpuUser.0", "-OvQ", "UCD-SNMP-MIB");
$idle = snmp_get($device, 'ssCpuIdle.0', '-OvQ', 'UCD-SNMP-MIB');

$proc = (100 - $idle);