Anwser

Solved!! Thanks to @IanAbbott

the header should be:

#include <linux/ktime.h>
#include <linux/timekeeping.h>

rather than <linux/time.h>.

More detail see discussion.

Original Question

I am writing a system call names sys_my_time.c, which will use getnstimeofday(). I have imported <linux/time.h>. The code like this:

#include <linux/kernel.h>
#include <linux/linkage.h>
#include <linux/time.h>

asmlinkage int sys_my_time() {
  struct timespec t;
  getnstimeofday(&t);
  // ...
  return 0;
}

But while compiling, the error shows:

CC      kernel/sys_my_time.o
kernel/sys_my_time.c: In function ‘sys_my_time’:
kernel/sys_my_time.c:8:3: error: implicit declaration of function ‘getnstimeofday’ [-Werror=implicit-function-declaration]
getnstimeofday(&t);
^
cc1: some warnings being treated as errors
scripts/Makefile.build:320: recipe for target 'kernel/sys_my_time.o' failed
make[1]: *** [kernel/sys_my_time.o] Error 1
Makefile:1029: recipe for target 'kernel' failed
make: *** [kernel] Error 2

I have no idea why the error happens.

P.S. compiling kernel V4.14.25 in Ubuntu 16.04