very confused about mesos task'cpu usage

[liuhui8]

hi~
there is a problem for me to calculation mesos tasks' cpu usage;
we are using prometheus+mesos_exporter to get mesos tasks' information,but when calculate cpu usage,what we have is:
mesos_agent_cpus_limit
mesos_agent_cpu_user_seconds_total
mesos_agent_cpu_system_seconds_total
i can‘t to calculate cpu usage with three params，i'll be very grateful if anyone could help
please ignoreme for my poor English ,thanks!!