How to measure the performance hit of changes in kernel code

[john k]

What is the optimal way to measure the performance hit a change in the
kernel brings to the system under normal conditions?
Are there tools for this job or a standard procedure kernel developers
follow or everyone comes up with his own metrics?
I know it sounds vague and maybe the measurement must be related with where
the change is on the kernel code.
Say for example that I implemented some extra security checks in the
copy_from_user function, or some other critical code. Whats the best way to
measure the performance hit it causes?
I have used measurements of the time a full kernel compile takes, but I dont
think this is the best metric.

Also, is there a test suite that somehow emulates actions of an average user
or another for an average web server? So that you can take average
measurements etc (In trying to establish an overall performance hit, so that
the metric may be unrelated to the part of the kernel code that is affected)
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