RFR: 8273482: Remove "foreground work" concept from WorkGang [v2]

[Per Liden]

On Thu, 9 Sep 2021 20:07:25 GMT, Per Liden <pliden at openjdk.org> wrote:

>> JDK-8237354 introduced the concept of "foreground work" in WorkGang, as a special case for use by the HeapDumper. I propose that we remove this code, since this special use case can be solved without the need for the concept of "foreground work" in WorkGang.
>> 
>> As far as I can tell, there's no reason why it must be the VM thread that takes on the task of writing the header, iterating over roots, etc. So, in VM_HeapDumper::work(), instead of checking if the current thread is the VM thread we can just check if the worker_id is non-zero. That way, a single worker thread will take on the task of writing the header, iterating over roots, etc, and all other worker threads will continue to call worker_loop().
>> 
>> Testing:
>>  - Passed Tier1-3
>>  - Passed multiple runs of test/hotspot/jtreg/serviceability/dcmd/gc/
>>  - Manually ran jcmd GC.heap_dump with various GCs enabled
>
> Per Liden has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove asserts

Thanks all for reviewing!

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PR: https://git.openjdk.java.net/jdk/pull/5410