RFR: 8368527: JMX: Add an MXBeans method to query GC CPU time [v11]

[Kevin Walls]

On Fri, 14 Nov 2025 11:29:48 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:
> 
>   Reduce GC coverage for trivial API test

Or, if we don't want to remove the GC specifics and go with what the framework sets, then we should revert that last delete, sorry, otherwise with will just not be run with all GCs.  

I was thinking it was more efficient to say nothing in the test, but can leave this to be done with specifics in the test if you like. 8-)

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PR Comment: https://git.openjdk.org/jdk/pull/27537#issuecomment-3533138317