RFR: 8299426: Heap dump does not contain virtual Thread stack references

[Serguei Spitsyn]

On Tue, 14 Nov 2023 21:54:06 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

> The change impelements dumping of unmounted virtual threads data (stack traces and stack references).
> Unmounted vthreads can be detected only by iterating over the heap, but hprof stack trace records (HPROF_FRAME/HPROF_TRACE) should be written before HPROF_HEAP_DUMP/HPROF_HEAP_DUMP_SEGMENT.
> HeapDumper supports segment dump (parallel dump to separate files with subsequent file merge outside of safepoint), the fix switches HeapDumper  to always use segment dump: 1st segment contains only non-heap data, other segments are used for dumping heap objects. For serial dumping single-threaded dumping is performed, but 2 segments are created anyway.
> When HeapObjectDumper detects unmounted virtual thread, it writes HPROF_FRAME/HPROF_TRACE records to the 1st segment ("global writer"), and writes thread object (HPROF_GC_ROOT_JAVA_FRAME) and stack references (HPROF_GC_ROOT_JAVA_FRAME/HPROF_GC_ROOT_JNI_LOCAL) to the HeapObjectDumper segment.
> As parallel dumpers may write HPROF_FRAME/HPROF_TRACE concurrently and VMDumper needs to write non-heap data before heap object dumpers can write virtual threads data, writing to global writer is protected with DumperController::_global_writer_lock.
> 
> Testing: run tests which perform heap dump (in different scenarios):
>  - test/hotspot/jtreg/serviceability
>  - test/hotspot/jtreg/runtime/ErrorHandling
>  - test/hotspot/jtreg/gc/epsilon
>  - test/jdk/sun/tools/jhsdb

src/hotspot/share/services/heapDumper.cpp line 2528:

> 2526: 
> 2527:     // At this point, all fragments of the heapdump have been written to separate files.
> 2528:     // We need to merge them into a complete heapdump and write HPROF_HEAP_DUMP_END at that time.

I like how the code in `VM_HeapDumper::work` function was re-arranged by removing some duplication and making more common vs conditional lines.

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PR Review Comment: https://git.openjdk.org/jdk/pull/16665#discussion_r1409954292