RFR: 8368527: JMX: Add an MXBeans method to query GC CPU time [v2]

[Alan Bateman]

On Sun, 5 Oct 2025 13:00:44 GMT, Jonas Norlinder <duke at openjdk.org> wrote:

>> Hi all,
>> 
>> This PR augments the CPU time sampling measurement capabilities that a user can perform from Java code with the addition of `MemoryMXBean.getGcCpuTime()`. With this patch it will be possible for a user to measure process and GC CPU time during critical section or iterations in benchmarks to name a few. This new method complements the existing `OperatingSystemMXBean.getProcessCpuTime()` for a refined understanding.
>> 
>> `CollectedHeap::gc_threads_do` may operate on terminated GC threads during shutdown, but thanks to JDK-8366865 by @walulyai we can piggyback on the new `Universe::is_shutting_down`. I have implemented a stress-test `test/jdk/java/lang/management/MemoryMXBean/GetGcCpuTime.java` that may identify reading CPU time of terminated threads. Synchronizing on `Universe::is_shutting_down` and `Heap_lock` resolves this problem.
>> 
>> FWIW; To my understanding we don't want to add a `Universe::is_shutting_down` check in gc_threads_do as this may introduce a performance penalty that is unacceptable, therefore we must be careful about the few places where external users call upon gc_threads_do and may race with a terminating VM.
>> 
>> Tested: test/jdk/java/lang/management/MemoryMXBean/GetGcCpuTime.java, jdk/javax/management/mxbean hotspot/jtreg/vmTestbase/nsk/monitoring on Linux x64, Linux aarch64, Windows x64, macOS x64 and macOS aarch64 with release and fastdebug.
>
> Jonas Norlinder has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Move from j.l.management to com.sun.management etc.

src/jdk.management/share/classes/com/sun/management/MemoryMXBean.java line 45:

> 43:      * GC cycle occurred. This method returns {@code -1} if the
> 44:      * platform does not support this operation or if called during
> 45:      * shutdown.

Now that the proposal is a JDK-specific management interface it means that this method can say something about how it relates to, or how it might be used with, OperatingSystemMXBean::getProcessCpuTime. 

I'm also wondering about ThreadMXBean::isThreadCpuTimeSupported. Is it possible for this to return false but getTotalGcCpuTime to return a value >= 0. The former concerns Java threads so it might not have any connection to this new method which concerns CPU usage by non-Java threads, but someone is bound to ask.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2405893440