How can I detect if a reboot happened

I come accross some unexpected auto reboot and no vmcore left. So I want to catch whenever a kernel reboot happened and who call this. As I thought before, when I input a reboot command, then the SYSCALL_DEFINE4(reboot) will be invoked and then kernel goes into the function kernel_restart. But the fact was, when I used kprobe to trace kernel_restart and entered reboot，nothing was caught ---- I've already tested on some other kernel function such as do_sys_open and ksys_umount and they all worked fine so at least my use of kprobe is not wrong. Now I wonder that is there any other way in kernel to call a reboot?