Is CPU % per core or for the whole CPU?
Right now my Linode is reaching 90% CPU usage at least once a day. Is this 90% of one core or 90% of all of my Linode's cores? My Linode has 8 cores.
The usage you're seeing refers to only one core. Because your plan has 8 cores, full usage of the processor would be reported as 800%, so at present your system is using only around 11% of its available CPU resources. Our recommendation is to keep your usage under 80% of its overall available CPU resources, or in your case 640%. This general guideline will help keep your Linode, as well as others on the same physical host as yours, running smoothly. You can set an alert that will let you know when your CPU usage is above a certain threshold in your Linode's Dashboard, under the "Settings" tab.
In the newer Cloud Manager, you can follow these instructions to change the alert threshold values.