procfs
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process stacks
Some good info here https://ops.tips/blog/using-procfs-to-get-process-stack-trace/
For anybody else finding this, the link is dead but it's archived here: https://web.archive.org/web/20220604093308/https://ops.tips/blog/using-procfs-to-get-process-stack-trace/.
Also it's referring to /proc/<pid>/stack, which is the kernel stack trace. I've been working with thread stacks recently, which are different (and have been removed from all the places I can find in /proc over time; the numbers are still there but they're all 0).