RFR: 8303242: ThreadMXBean issues with virtual threads

[Mandy Chung]

On Mon, 27 Feb 2023 12:23:09 GMT, Alan Bateman <alanb at openjdk.org> wrote:

> This PR covers a number of issues with j.l.management.ThreadMXBean, and the JDK-specific extension c.s.management.ThreadMXBean, when there are virtual threads in use.
> 
> As background, ThreadMXBean was re-specified in Java 19 to support the monitoring and management of platform threads. It does not support virtual threads as their potential number, and the need to find a thread by id, does not make sense for this API. At the same time, JDK 19 introduced an alternative implementation of virtual threads for Zero and ports without continuations support in the VM. This alternative implementation of virtual threads means a JavaThread per virtual thread and so requires filtering to ensure that the API behaves as specified. For the initial implementation, the filtering was done in the ThreadMXBean implementation. That works for most functions but not for getThreadXXXTime(long[]) and getThreadAllocatedBytes(long[]) where the filtering needs to be pushed down to the management code.
> 
> The changes in this PR move the filtering to the management functions (jmm_XXX) so they only return information about platform threads. There are some minor adjustments to the API docs (see linked CSR). Test coverage is expanded as we didn't include tests for c.s.management.ThreadMXBean with virtual threads in JDK 19.
> 
> Testing tier1-3 (jdk_management test group is in test/jdk/:tier3), plus sanity checking that --with-jvm-variants=minimal builds as some of this code is not compiled in with minimal VM builds.

The "Thread CPU time" section in the class spec of ThreadMXBean can also be clarified.

src/java.management/share/classes/java/lang/management/ThreadMXBean.java line 529:

> 527:     /**
> 528:      * Tests if the Java virtual machine implementation supports CPU time
> 529:      * measurement for any platform thread.

This change can also apply in `@return` (line 538)

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()`.

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