[ruby-core:115496] [Ruby master Bug#20021] TestGCCompact#test_moving_hashes_down_size_pools is flaky

Issue #20021 has been updated by mame (Yusuke Endoh).

Status changed from Open to Closed

Closed by commit:8427a8a655e2a04bfdc6a645ec967405d3617137

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Bug #20021: TestGCCompact#test_moving_hashes_down_size_pools is flaky
https://bugs.ruby-lang.org/issues/20021#change-105425

* Author: kjtsanaktsidis (KJ Tsanaktsidis)
* Status: Closed
* Priority: Normal
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
----------------------------------------
The test `TestGCCompact#test_moving_hashes_down_size_pools is flaky` is, I believe, flaky, and sometimes fails with this message:

```
  1) Failure:
TestGCCompact#test_moving_hashes_down_size_pools [/home/chkbuild/chkbuild/tmp/build/20231112T183003Z/ruby/test/ruby/test_gc_compact.rb:442]:
Expected 499 to be >= 500.
```

I started looking at this because when https://github.com/ruby/ruby/pull/8858 was merged, rubyci began failing on Ubuntu 22.04 with this message (https://rubyci.s3.amazonaws.com/ubuntu2204/ruby-master/log/20231112T183003Z.fail.html.gz). I was able to reproduce this failure after _very_ carefully reproducing the ruibyci testing environment (same EC2 instance type, and I actually had to install _exactly_ the same kernel version as well!).

After debugging the test, what I discovered is that the _last_ hash that was modified in the test is actually on the machine stack when `GC.verify_compaction_references` is called, which means it gets pinned and cannot be moved; thus, the test fails because only 499 out of 500 objects got moved! The hash's VALUE is "on the stack" in the sense that it's in a memory location below `%rsp`, but it is _NOT_ actually live in any C local variable. The stack address I found the VALUE in is part of the frame of `gc_start`, but `gc_start` obviously doesn't hold a reference to the hash from the test. Rather, it's left-behind in uninitialized memory.

I think this is made more likely because of how GCC wound up compiling the `gc_start` function. It seems to have allocated quite a lot of stack space - 440 bytes of it - and very little of it seems to be used for local variables. Thus, a lot of this memory just contains assorted tidbits from previous VM execution state.

This patch works around the issue, at least on my testbench, by running the code which touches the test hashes in a separate fiber. Each fiber gets its own machine stack, so references to the to-be-moved hashes shouldn't leak into the main stack, and this fiber's stack should be out of scope when the compaction is run, so it should get freed. https://github.com/ruby/ruby/pull/9040

The other approach I considered is counting the number of pinned objects, and asserting that (moved + pinned) >= 500 instead of just moved >= 500. However, it's very likely that there will be a pinned hash (or string, etc, for the other, similar test cases), so this would make the test pass even when it should not, I think.

Also, since this these tests were skipped on Solaris because of this bug, I have unskipped them (although it seems we no longer have solaris on ruby ci, so I guess it doesn't matter).



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