RFR: 8273482: Remove "foreground work" concept from WorkGang

[Kim Barrett]

On Wed, 8 Sep 2021 09:56:21 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

Looks good.

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Marked as reviewed by kbarrett (Reviewer).

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