rem: fromThread->kernel_time += now - fromThread->last_time;
add: if(fromThread->last_time_type == KERNEL_TIME) fromThread->kernel_time += now - fromThread->last_time; else fromThread->user_time += now - fromThread->last_time;
context: context_switch(struct thread *fromThread, struct thread *toThread){	bigtime_t now;	// track kernel time	now = system_time();	fromThread->kernel_time += now - fromThread->last_time;	toThread->last_time = now;	toThread->cpu = fromThread->cpu;	fromThread->cpu = NULL;	arch_thread_set_current_thread(toThread);	arch_thread_context_switch(fromThread, toThread);}