RFR: 8303242: ThreadMXBean issues with virtual threads [v2]

[Alan Bateman]

On Tue, 28 Feb 2023 21:06:19 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> 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 five additional commits since the last revision:
>> 
>>  - Clarify Thread CPU time seciton of spec
>>  - Merge
>>  - Fix minimal build
>>  - Fix minimal build
>>  - Initial commit
>
> test/jdk/java/lang/management/ThreadMXBean/VirtualThreads.java line 258:
> 
>> 256:             long tid = Thread.currentThread().threadId();
>> 257:             long cpuTime = bean.getThreadCpuTime(tid);
>> 258:             assertEquals(-1L, cpuTime);
> 
> Am I correct that `getThreadCpuTime(tid)` returns -1 for the current thread is a virtual thread whereas `getCurrentThreadCpuTime` throws UOE in the current implementation?
> 
> `getCurrentThreadCpuTime` is specified to be equivalent to calling `getThreadCpuTime(Thread.currentThread().threadId()`.

We didn't get this quite right in JDK 19 but I think I've fixed all those issues now.  So assuming the VM supports CPU time for all platform threads, it means:

- isThreadCpuTimeEnabled and isCurrentThreadCpuTimeSupported will return true.
- If getThreadCpuTime(long) is called with the thread ID of a virtual thread then -1 will be returned.
- If getCurrentThreadCpuTime() is called from a virtual thread then -1 will be  returned.

I did another pass over the API docs, update the "Thread CPU time" section, so I hope it is clearer now.

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PR: https://git.openjdk.org/jdk/pull/12762