Part of the problem is that the kernel varies so much from release to release, often in un-git-bisectable ways.
I run regression tests of the overhead of the perf_event syscalls. You can see some results here.
There are often wild swings in the results of 5-10% from kernel release to release, but it just seems to be "noise". I've tried bisecting but it just gets you nowhere. When I went to the kernel devs they just said it's probably different cache layout and similar affects from unrelated pieces of the code. So sadly regressions are just completely lost in noise unless they are *major* slowdowns.
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