RFR: 8360024: Reorganize GC VM operations and implement is_gc_operation

[David Holmes]

On Thu, 19 Jun 2025 11:26:26 GMT, Jonas Norlinder <duke at openjdk.org> wrote:

> [JDK-8359110](https://bugs.openjdk.org/browse/JDK-8359110) introduces a need to be able to categorize VM operations to be able to track GC activity in the VM thread for CPU time tracking. This patch will reorganize the shared GC VM operations in gcVMOperations.hpp to make a distinction between GC operations and serviceability/CDS etc. operations performed in the VM thread for the GC.
> 
> Additionally the ability to query `is_gc_operation()` is introduced in the base class `VM_Operation` and overridden in appropriate VM operations. This provides a clear and efficient interface to query the category of a VM operation.
> 
> Proposed changes in the shared class hierarchy:
> 
> 
> //  VM_Operation
> //    VM_Heap_Sync_Operation
> //      VM_GC_Operation
> //        VM_GC_Collect_Operation
> //          VM_SerialGCCollect
> //          VM_ParallelGCCollect
> //          VM_CollectForAllocation
> //            VM_SerialCollectForAllocation
> //            VM_ParallelCollectForAllocation
> //          VM_CollectForMetadataAllocation
> //        VM_GC_Service_Operation
> //          VM_GC_HeapInspection
> //      VM_Verify
> //      VM_PopulateDynamicDumpSharedSpace
> //      VM_PopulateDumpSharedSpace
> 
> 
> Renaming `VM_GC_Sync_Operation` to `VM_Heap_Sync_Operation` to make it clear that not all sub-classes is related to GC (shout-out to @kstefanj / @stefank for that suggestion). While `VM_GC_HeapInspection` is a serviceability operation it attempts to trigger a collection, hence it is put underneath `VM_GC_Collect_Operation->VM_GC_Service_Operation`. However only classes that inherits from `VM_GC_Collect_Operation` are considered to return `true` for `is_gc_operation()` as `VM_GC_HeapInspection` is considered to not belong to "GC activity".
> 
> ZGC and Shenandoah do not use sub-classing of `VM_Heap_Sync_Operation`, but their respective base classes and special classes for GC activity are marked as returning `true` for `is_gc_operation()`.
> 
> The only `is_XX_operation` that is specified in `VM_Operation` is `is_gc_operation` to not clutter with cases that are not used (`is_gc_operation` will be used by JDK-8359110). That being said, these behavioral queries may be extended in case we want to explore CPU time tracking beyond the GC component.
> 
> _Note: PR for JDK-8359110 already contains a proposal for `is_gc_operation` but it was deemed that it was more appropriate to bring along that change during reorganization of GC VM operations since implementing it at the correct places without this change would require a lot...

This looks quite reasonable to me. Obviously GC folk get final approval.

One minor nit.

src/hotspot/share/gc/shared/gcVMOperations.hpp line 188:

> 186:                     GCCause::_heap_inspection /* GC Cause */,
> 187:                     0 /* total full collections, dummy, ignored */,
> 188:                     request_full_gc), _out(out), _full_gc(request_full_gc),

Please re-align the arguments.

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Marked as reviewed by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25896#pullrequestreview-2944528759
PR Review Comment: https://git.openjdk.org/jdk/pull/25896#discussion_r2158024854