# frequenza aggiornamento kernel
cat /boot/config-`uname -r` | grep HZ

# pcpu espressa da ps
total_time = pp->utime + pp->stime;
if(include_dead_children) total_time += (pp->cutime + pp->cstime);
seconds = seconds_since_boot - pp->start_time / Hertz;
if(seconds) pcpu = (total_time * 1000ULL / Hertz) / seconds;
if (pcpu > 999U)
return snprintf(outbuf, COLWID, "%u", pcpu/10U);
return snprintf(outbuf, COLWID, "%u.%u", pcpu/10U, pcpu%10U);

# process time raccolti dal kernel in /proc/pid/stat
http://stackoverflow.com/questions/16726779/total-cpu-usage-of-an-application-from-proc-pid-stat

# process time in /proc dettagliati
utime %lu   (14) Amount of time that this process has been scheduled in user mode, measured in clock ticks divide by sysconf(_SC_CLK_TCK)).  This includes guest time, guest_time (time spent running a virtual CPU, see below), so that applications that are not aware of the guest time field do not lose that time from their calculations.
stime %lu   (15) Amount of time that this process has been scheduled in kernel mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK)).
cutime %ld  (16) Amount of time that this process's waited-for children have been scheduled in user mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK)).  (See also times(2).)  This includes guest time, cguest_time (time spent running a virtual CPU, see below).
cstime %ld  (17) Amount of time that this process's waited-for children have been scheduled in kernel mode, measured in clock ticks (divide by sysconf(_SC_CLK_TCK)).