How do I get the total CPU usage of an application from /proc/pid/stat?

Question

Preparation

To calculate CPU usage for a specific process you’ll need the following:

/proc/uptime
- #1 uptime of the system (seconds)
/proc/[PID]/stat
- #14 utime – CPU time spent in user code, measured in clock ticks
- #15 stime – CPU time spent in kernel code, measured in clock ticks
- #16 cutime – Waited-for children’s CPU time spent in user code (in clock ticks)
- #17 cstime – Waited-for children’s CPU time spent in kernel code (in clock ticks)
- #22 starttime – Time when the process started, measured in clock ticks
Hertz (number of clock ticks per second) of your system.
- In most cases, getconf CLK_TCK can be used to return the number of clock ticks.
- The sysconf(_SC_CLK_TCK) C function call may also be used to return the hertz value.

First we determine the total time spent for the process:

total_time = utime + stime

We also have to decide whether we want to include the time from children processes. If we do, then we add those values to total_time:

total_time = total_time + cutime + cstime

Next we get the total elapsed time in seconds since the process started:

seconds = uptime - (starttime / Hertz)

Finally we calculate the CPU usage percentage:

cpu_usage = 100 * ((total_time / Hertz) / seconds)