Execution time of a specific segment of code in the kernel

For details about how to obtain the execution time of a specific segment of code in the kernel-Linux general technology-Linux programming and kernel information, see the following. Under 2.6.22, I modified the system to call the outermost kernel functions sys_read () and sys_write () corresponding to read () and write () to extend the read/write functions, now I want to test the extra time consumption caused by the extended functions.
My idea is to take the current time at the start of the function code, and then take the end time, then calculate the difference, and accumulate. The method is as follows:
Unsigned long starttime, finishtime;
Startime = jiffies/HZ; // The start time is the current time.
Finishtime = jiffies/HZ; // The end time is the current time.
Printk ("syc_seconds = % d \ nNow totaluse = % d", finishtime-starttime, totalpolicime );
(The totalpolicime variable is placed outside the function to become a global variable)
However, the recorded time in this method is always 0.
Is there a problem with my thoughts? In the kernel, how should we get the current time at a certain time point and record the execution time of a piece of code?
Hope you can solve this problem. thank you very much!