pthread_make_periodic_np and scheduling

[Arnold Radtke <Arnold.Radtke@Ferrari.DE>]

I have a rt thread which generates periodic interrupts each 125us,
created with phtread_make_periodic_np. If this thread becomes active, it
wakes up one of 32 other rt test threads with 
pthread_kill(thread[i],RTL_SIGNAL_WAKEUP). One thread is running for
less than 125us and suspends(phtread_suspend_np(pthread_self()). If I
measure the time the thread is running with gethrtime and save the max
value which is reported if the module is reloaded, I got max time of one
thread about 192us. My test thread does only execute some dummy code for
using CPU time (< 10us). Thats definitly because I check at each
interrupt in the periodic task if one of my test thread is interrupted.
What happens in the 192us ? 

Thanks for any ideas.
Arnold