Why my kernel log is not showing the latest output?

I'm coding a simple kernel module, in Ubuntu 17.04, that takes a string and prints it in the kernel log.

#include<linux/module.h>#include<linux/init.h>#include<linux/moduleparam.h>char* mystring = "hello world";module_param(mystring ,charp ,S_IRUSR | S_IWUSR);void display(void){printk(KERN_ALERT "%s" ,mystring);}static int hello(void){//printk(KERN_ALERT "hello module");display();return 0;} static void bye(void){printk(KERN_ALERT "bye");}module_init(hello);module_exit(bye);

I run command make and then when I run insmod test.ko mystring="blahblahblah", the module will be inserted correctly but when I run dmesg it doesn't show the blahblahblah.

After I run rmmod test.ko and dmseg the expression blahblahblah will appear in the terminal. When I run insmod test.ko mystring="blahblahblah" again and then dmesg the blahblahblah will be printed.

what is the problem exactly? Is it my problem or the system?