RFR: 8297427: Avoid keeping class loaders alive when executing ClassLoaderStatsVMOperation

[Stefan Johansson]

On Tue, 22 Nov 2022 20:54:54 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> Please review this change to avoid keeping classes alive only due to the `ClassLoaderStatsVMOperation`.
> 
> **Summary**
> The `ClassLoaderStatsVMOperation` is gathering statistics about the active class loaders in a safepoint. The way the `ClassLoaderDataGraph` is iterated will keep the class loaders live. This is not really needed since everything is done in a safepoint and nothing needs to be explicitly kept alive. This has not been a problem prior to concurrent class unloading in ZGC. With fully concurrent class unloading a `ClassLoaderStatsVMOperation` can occur during a collection and more classes than needed might be kept alive. This could in turn lead to premature Metaspace OOM. 
> 
> The solution is to not keep the class loaders alive due to the iteration in `ClassLoaderStatsVMOperation`.
> 
> **Testing**
> * Added a new test that covers the two different ways a class could previously be kept alive by the VM operation. The test passes after the fix but failed before.  
> * Mach5 tier 1-3

Reverted the use of templates after some internal discussions. Also added a print of the object in the test to ensure the class is kept alive even if `-Xcomp` is used and ergonomically GCs are triggered before the one invoked by the test.

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PR: https://git.openjdk.org/jdk/pull/11300