RFR: 8359110: Log accumulated GC and process CPU time upon VM exit [v17]

[Albert Mingkun Yang]

On Thu, 3 Jul 2025 07:40:56 GMT, Jonas Norlinder <duke at openjdk.org> wrote:

>> Add support to log CPU cost for GC during VM exit with `-Xlog:gc+cpu`.
>> 
>> 
>> [2,430s][info][gc,cpu] GC CPU usage: 22,87% (Process: 26,8926s GC: 6,1491s)
>
> Jonas Norlinder has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 27 commits:
> 
>  - Move call to print_tracing_info
>  - Merge branch 'master' into gc_cpu_time
>  - Remove static and move string_dedup_cpu_time into elapsed_gc_cpu_time
>  - Fixes after @albertnetymk review
>  - Change output per discussion
>  - Make stop protected, remove integer divison, remove unused method
>  - Update closure name
>  - vtime -> cpu_time and _vm_vtime -> _vmthread_cpu_time
>  - Fixes after review from @stefank
>  - Merge branch 'master' of github.com:JonasNorlinder/openjdk_jdk into gc_cpu_time
>  - ... and 17 more: https://git.openjdk.org/jdk/compare/c75df634...f9c3aa64

src/hotspot/share/gc/shared/collectedHeap.cpp line 206:

> 204: class CPUTimeThreadClosure : public ThreadClosure {
> 205: private:
> 206:   volatile jlong _cpu_time = 0;

There is no need for `volatile`. `gc_threads_do` runs on the calling thread, not on threads we are iterating on.

src/hotspot/share/gc/shared/collectedHeap.cpp line 226:

> 224:   if (string_dedup_cpu_time == -1) {
> 225:     return -1;
> 226:   }

Can probably move this to the very beginning for clearer separation: gc-time comes from three sources -- vmthread, string-dedup and gc-threads.

src/hotspot/share/gc/shared/cpuTimeScope.inline.hpp line 57:

> 55:   if (UsePerfData) {
> 56:     CPUTimeCounters::get_instance()->update_counter(CPUTimeGroups::CPUTimeType::vm, end);
> 57:   }

This class (name/interface) seems fairly generic, but the underlying impl is tightly coupled to vmthread. IOW, one can't use this class other than on vmthread. Maybe this class should be inside vmThread.cpp.

src/hotspot/share/runtime/cpuTimeCounters.hpp line 56:

> 54: 
> 55: class CPUTimeCounters: public CHeapObj<mtServiceability> {
> 56:   friend class CPUTimeScope;

I wonder if we avoid this. The public APIs should be enough.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25779#discussion_r2182127690
PR Review Comment: https://git.openjdk.org/jdk/pull/25779#discussion_r2182131415
PR Review Comment: https://git.openjdk.org/jdk/pull/25779#discussion_r2182146520
PR Review Comment: https://git.openjdk.org/jdk/pull/25779#discussion_r2182156134