RFR: 8299426: Heap dump does not contain virtual Thread stack references [v2]

[Alex Menkov]

> 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

Alex Menkov has updated the pull request incrementally with two additional commits since the last revision:

 - feedback
 - prepare_parallel_dum

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16665/files
  - new: https://git.openjdk.org/jdk/pull/16665/files/43268104..e1adc026

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16665&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16665&range=00-01

  Stats: 19 lines in 1 file changed: 8 ins; 3 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/16665.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16665/head:pull/16665

PR: https://git.openjdk.org/jdk/pull/16665