The GNOME monitor poll /proc/stats at regular interval to compute average usage. This file is updated by the kernel. The values comes from sampling that the kernel does for accounting various process.
How could we compute the CPU usage history from the actual kernel trace? We only have to look at sched_schedule events. The event inform us that in a certain point in time, the kernel switched the task on a given CPU. If the new task has PID zero, we know that's the idle thread from the kernel. By splitting running and idle time in fixed intervals, we can compute CPU average usage.
I show one small examples done on my computer that is dual core. First, we see the execution of three cpuburn process started with one second of delay. We can see the result in the control flow viewer in LTTV, and the resulting CPU usage view.
We see a peek at the beginning of the trace. This is likely to be related to the metadata flushing when a trace is started. The two steps after are related to each cpuburn process, and we see that when the third process is started, the CPU usage stays at 100%.
For testing, I added scripts to generate those traces automatically on your computer. It's written in Java, and you will need some extra packages to run it, see the README file. Get the code on github:
https://github.com/giraldeau/flightbox
Happy hacking!
