Function fall_through_count 

pub fn fall_through_count() -> u64

How many HotFn calls, on any thread, missed the jump table since the last patch was applied (see last_call_fell_through). Reset to zero by every successful apply_patch.

The count includes the benign misses of calls made from patch-image code, so after a patch to an app with nested components it is normally non-zero. It stays a plain count in this crate; telling a stale-code miss from a benign one needs the missed key (whose address range names the calling image), which missed_keys records.