In rolling forward the SetMaxHeap patches, I discovered that, as a result of the fix to #19112, if ReadMemStats is called in a tight loop, then it might keep stomping on other goroutines trying to stop the world by reacquiring worldsema before others get the chance. The worst case is if it stomps on a goroutine trying to stop the world for either starting or stopping a GC, which messes with pacing significantly.

The good news is, ReadMemStats is known to be a heavyweight operation, and so isn't called often. This is really only a problem if it's called in a tight loop. In the reproduction "tight loop" consisted of a single goroutine performing a large allocation, calling ReadMemStats, then doing some arithmetic and predictable branches. Adding in even a 1 ms sleep is enough to resolve the issue. This case is more likely to appear in tests and examples, where the behavior will be unexpected.