RFR: 8337199: Add jcmd Thread.vthread_scheduler and Thread.vthread_pollers diagnostic commands [v3]

[David Holmes]

On Fri, 29 Nov 2024 11:12:22 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Adds `jcmd <pid> Thread.vthread_scheduler` to print the virtual thread scheduler and `jcmd <pid> Thread.vthread_pollers` to print the I/O pollers that support virtual threads doing blocking network I/O operations.
>> 
>> This is a subset of the diagnostics that we've had in the loom repo for a long time. @larry-cable proposed a PR recently ([pull/22121](https://github.com/openjdk/jdk/pull/22121)) to bring a version of same into main line but it was based on an older proposal. This new PR supplants that effort.
>> 
>> The jtreg failure handler is updated to execute Thread.vthread_scheduler, useful capture when a test fails or times out.
>
> Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8337199
>  - VThread*Dcmd::execute changes to use helper function
>  - Improve regex for matching poller String representation
>  - Improve vthread_pollers output
>  - Merge branch 'master' into JDK-8337199
>  - Initial commit

LGTM! Thanks

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Marked as reviewed by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/22414#pullrequestreview-2471545814