This switches tracking CPU usage to more accurately measure time in
user and kernel land using either the TSC or another time source.
This will also come in handy when implementing a tickless kernel mode.
As threads come and go, we can't simply account for how many time
slices the threads at any given point may have been using. We need to
also account for threads that have since disappeared. This means we
also need to track how many time slices we have expired globally.
However, because this doesn't account for context switches outside of
the system timer tick values may still be under-reported. To solve this
we will need to track more accurate time information on each context
switch.
This also fixes top's cpu usage calculation which was still based on
the number of context switches.
Fixes#6473
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
