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

[Alan Bateman]

On Thu, 28 Nov 2024 00:04:53 GMT, Chen Liang <liach 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.
>
> test/hotspot/jtreg/serviceability/dcmd/thread/VThreadCommandsTest.java line 2:
> 
>> 1: /*
>> 2:  * Copyright (c) 2023, 2024, Oracle and/or its affiliates. All rights reserved.
> 
> Suggestion:
> 
>  * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.

The feature + test were initially created in Oct 2023. It might look like a new test for main line but I think we are supposed to keep the original creation date.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22414#discussion_r1861620057