how to accurately measure processing time

Hi,

I writing a packet processor that takes one packet from 1 to n distinct
streams sequentially, and processes them one at a time. How can I
*accurately* measure the packet processing delay for each individual packet?

Each packet stream is continuous, but they are not synchronous, i.e. for
each stream, the arrival time of the first bit of each packet is different.

For example, when processing the first packet, I would like to know how
much time is spent with the actually processing of that packet....So I
don't want to include any time when the process gets swapped out etc. For
this reason, I don't want to simply get the system time before and after
the function.

I hope my question is clear, but please ask me to clarify if it isn't.

Thank you,
Ramy