changes to /proc files

I'm writing a little GPL app that needs to monitor some data in /proc. I
seem to recall (possibly incorrectly) that there was some magic way for a
program to be notified when a file in /proc changes, rather than having to
poll. I can't seem to find anything in Google.

Can anyone point me in the right direction?
What is the overhead in polling and reading a small (~500b) /proc file?

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