RFR: 8368527: JMX: Add an MXBeans method to query GC CPU time [v7]

[Alan Bateman]

On Fri, 24 Oct 2025 12:37:02 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:
> 
>   Fix link for getProcessCpuTime

src/java.management/share/classes/java/lang/management/MemoryMXBean.java line 274:

> 272:      * spent in garbage collection.
> 273:      *
> 274:      * <p> This is the CPU time used by all garbage collection

Starting this paragraph with "This is the CPU time ..." is a bit awkward. Can you try "The time spent in spent in garbage collection (GC) is the CPU time ...".

src/java.management/share/classes/java/lang/management/MemoryMXBean.java line 289:

> 287:      * in garbage collection is highly implementation dependent.
> 288:      * In the HotSpot Virtual Machine implementation reported
> 289:      * time will include relevant implementation-specific details such

"In the HotSpot Virtual Machine implementation reported time will include".  A suggestion to improve this is to change it to "In the HotSpot Virtual Machine, this time includes.."

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PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2461014442
PR Review Comment: https://git.openjdk.org/jdk/pull/27537#discussion_r2461027518