RFR: 8364638: Refactor and make accumulated GC CPU time code generic [v10]

[Albert Mingkun Yang]

On Tue, 19 Aug 2025 08:42:15 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 unused

src/hotspot/share/gc/shared/collectedHeap.cpp line 625:

> 623:       ClassLoaderDataGraph::print_on(&ls_trace);
> 624:     }
> 625:   }

I don't think this code movement should be done -- this calls back to `Universe` and CLDG printing is not necessarily tired to heap.

src/hotspot/share/services/cpuTimeUsage.cpp line 43:

> 41: public:
> 42:   virtual void do_thread(Thread* thread) {
> 43:     jlong cpu_time = os::thread_cpu_time(thread);

I guess a wrapper for `thread_cpu_time` can be created to group error-handling logic together, for all uses in this file.


jlong thread_cpu_time_or_zero(thread) {
  jlong cpu_time = os::..
  if (cpu_time == -1) {
    // mark-error
    return 0;
  }
  return cpu_time;
}

src/hotspot/share/services/cpuTimeUsage.hpp line 47:

> 45:     static jlong gc_threads();
> 46:     static jlong vm_thread();
> 47:     static jlong stringdedup();

I feel the API surface contains some redundancy.

For example, the GC-part API design exposes two ways to query and they are essentially the same -- for the sake of simplicity, I'd suggest keeping only one.

The calculation of `total` should be done by users of this API, I believe, when/if total is desirable.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/26621#discussion_r2284933253
PR Review Comment: https://git.openjdk.org/jdk/pull/26621#discussion_r2284962585
PR Review Comment: https://git.openjdk.org/jdk/pull/26621#discussion_r2284952784