Perf command not support energy when passing process ID

Question

I want to attach perf to a process and measure the package energy. As you can see below, the example shows that power/energy-pkg/ works fine.

$ perf stat -a -e power/energy-pkg/ -- sleep 1

 Performance counter stats for 'system wide':

              1,71 Joules power/energy-pkg/

       1,019749321 seconds time elapsed

The C code gets the PID at the beginning and stops until a SIGCONT is received. With the following commands, I launch the program in background and grab the PID. Then I continue the execution and run perf with the PID.

$ ./app &
$ PID=`pidof app`
$ kill -SIGCONT $PID
$ perf stat -a -e power/energy-pkg -p $PID
   Performance counter stats for process id '2590':

    <not supported> Joules power/energy-pkg/

      34,002655205 seconds time elapsed

As you can see the energy number is invalid. Any idea about this behavior?