RFR: JDK-8321565: [REDO] Heap dump does not contain virtual Thread stack references

[Alex Menkov]

On Mon, 11 Dec 2023 06:05:52 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Original fix for JDK-8299426 (Heap dump does not contain virtual Thread stack references, #16665) caused failures of new test (added while #16665 was under review):
>> test/hotspot/jtreg/compiler/c2/TestReduceAllocationAndHeapDump.java in many tears and was reverted.
>> 
>> Segmented heap dump assumes "merge" stage is executed outside of safepoint (to not block the VM), but heap dump may happen during safepoint (and TestReduceAllocationAndHeapDump.java test provoke the case).
>> The change contains original fix for JDK-8299426 ("[original fix](https://github.com/openjdk/jdk/commit/bdbf768eafa86e0007aca4188e0567693afe9071)") and removes asserts from HeapMerger ([allow heapdump in safepoints](https://github.com/openjdk/jdk/commit/44670ca4bf55dd2a5f1f44686758844aed68937e)).
>> 
>> Run tier1-3 and heapdump-related tests.
>
> src/hotspot/share/services/heapDumper.cpp line 1984:
> 
>> 1982: // user space.
>> 1983: void DumpMerger::merge_file(char* path) {
>> 1984:   assert(!SafepointSynchronize::is_at_safepoint(), "merging happens outside safepoint");
> 
> This might fix the failure but why is this restriction in place to begin with? This seems like a very expensive operation to be performing by the VMThread during a safepoint!

Main idea of segmented heap dump feature (JDK-8306441, #13667) is to divide heap dump process to 2 phases - dump to segment files (this phase is executed at safepoint) and merge of the segment files (this one is supposed to be executed outside of safepoint, so VM is not blocked in this phase). And this asserts were added to ensure it works as desired.
I think merge stage can be performed on the current thread without VMOperation, but I don't know all possible consequences of the change.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17040#discussion_r1423261940