RFR: 8364638: Refactor and make accumulated GC CPU time code generic [v11]

[Jonas Norlinder]

On Wed, 20 Aug 2025 09:00:30 GMT, Jonas Norlinder <duke at openjdk.org> wrote:

>> Hi all,
>> 
>> This PR refactors the newly added GC CPU time code from [JDK-8359110](https://bugs.openjdk.org/browse/JDK-8359110).
>> 
>> As a stepping-stone to enable consolidation of CPU time tracking in e.g. hsperf counters and GCTraceCPUTime and to have a unified interface for tracking CPU time of various components in Hotspot this code can be refactored. This PR introduces a new interface to retrieve CPU time for various Hotspot components and it currently supports:
>> 
>> CPUTimeUsage::GC::total() // the sum of gc_threads(), vm_thread(), stringdedup()
>> 
>> CPUTimeUsage::GC::gc_threads()
>> CPUTimeUsage::GC::vm_thread()
>> CPUTimeUsage::GC::stringdedup()
>> 
>> CPUTimeUsage::Runtime::vm_thread()
>> 
>> 
>> I moved `CPUTimeUsage` to `src/hotspot/share/services` since it seemed fitting as it housed similar performance tracking code like `RuntimeService`, as this is no longer a class that is only specific to GC.
>> 
>> I also made a minor improvement in the CPU time logging during exit. Since `CPUTimeUsage` supports more components than just GC I changed the logging flag to from `gc,cpu` to `cpu` and created a detailed table:
>> 
>> 
>> [71.425s][info][cpu   ] === CPU time Statistics =============================================================
>> [71.425s][info][cpu   ]                                                                             CPUs
>> [71.425s][info][cpu   ]                                                                s       %  utilized
>> [71.425s][info][cpu   ]    Process
>> [71.425s][info][cpu   ]      Total                                             1616.3627  100.00      22.6
>> [71.425s][info][cpu   ]      VM Thread                                            5.2992    0.33       0.1
>> [71.425s][info][cpu   ]      Garbage Collection                                  83.7322    5.18       1.2
>> [71.425s][info][cpu   ]        GC Threads                                        82.7671    5.12       1.2
>> [71.425s][info][cpu   ]        VM Thread                                          0.9651    0.06       0.0
>> [71.425s][info][cpu   ] =====================================================================================
>> 
>> 
>> Additionally, if CPU time retrieval fails it should not be the caller's responsibility to log warnings as this would bloat the code unnecessarily. I've noticed that `os` does log a warning for some methods if they fail so I continued on this path.
>
> Jonas Norlinder has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove GCStatistics, add thread_cpu_time_or_zero, move GC/heap log to Universe::before_exit

I had an offline discussion with @albertnetymk (thanks for bringing this up!) and we both agree that the current definition of `CPUTimeUsage::Runtime::vm_thread()` may be misread. As a consequence, we believe that expanding this interface to include more components would be best served by creating a separate PR that can serve as an explicit discussion for the layout. As such I'm removing `CPUTimeUsage::Runtime` from this PR.

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PR Comment: https://git.openjdk.org/jdk/pull/26621#issuecomment-3210275213