runtime: ReadMemStats called in a loop may prevent GC

[mknyszek]

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.

[mknyszek]

CC @aclements @dr2chase

Fix is already uploaded, just need to associate it with this issue.

[mknyszek]

FTR: the fix is to make starting the world hand off worldsema directly to one of the waiting goroutines. This makes the behavior around the semaphore more fair, and helps prevent the situation described in the original post.

[gopherbot]

Change https://golang.org/cl/243977 mentions this issue: runtime: release worldsema with a direct G handoff

[odeke-em]

Thank you for reporting this @mknyszek and thank you for having the fix steady and ready! Could we perhaps include a footnote BUG in the runtime package about this, or perhaps also in the release notes as an acknowledgement of the bug?