骨骼面试题一道

Write a C program which measures the the speed of a context switch on a UNIX/Linux system.
这个应该怎么回答，感觉在用户态只能近似测量。

I'd like to share my understanding upon this problem: total time which is easy to measure is equal to the context switch time duration plus the time threads need to perform tasks, if we can assume thread task consume consist time, then concurrency time consuming minus totally time eclipse without concurrency will lead to the answer. I'm new to OS fields, let me know if I'm wrong or better ways exist.