RFR: 8365238: 'jfr' feature requires 'services' with 'custom' build variant

[Magnus Ihse Bursie]

On Mon, 11 Aug 2025 10:55:44 GMT, Francesco Andreuzzi <duke at openjdk.org> wrote:

> In this PR I add an `autoconfigure` check to make sure that `jfr` is not built without the feature `services`, which would lead to the following error:
> 
> /jdk/src/hotspot/share/jfr/periodic/jfrPeriodic.cpp: In member function ‘virtual void VM_GC_SendObjectCountEvent::doit()’:
> /jdk/src/hotspot/share/jfr/periodic/jfrPeriodic.cpp:402:5: error: ‘ObjectCountEventSender’ has not been declared
>   402 |     ObjectCountEventSender::enable_requestable_event();
>       |     ^~~~~~~~~~~~~~~~~~~~~~
> /jdk/src/hotspot/share/jfr/periodic/jfrPeriodic.cpp:404:5: error: ‘ObjectCountEventSender’ has not been declared
>   404 |     ObjectCountEventSender::disable_requestable_event();
>       |     ^~~~~~~~~~~~~~~~~~~~~~
> 
> 
> To reproduce:
> 
> sh configure --with-jvm-variants=custom --with-conf-name=cstm --enable-jvm-feature-jfr --enable-jvm-feature-serialgc
> make -j hotspot CONF_NAME=cstm

I think it is not that simple, unfortunately. I tried changing the ifdef but ran into additional issues. In gcTrace.cpp we have:

  virtual void do_cinfo(KlassInfoEntry* entry) {
    if (should_send_event(entry)) {
      ObjectCountEventSender::send(entry, _timestamp);
    }
  }

 private:
  bool should_send_event(const KlassInfoEntry* entry) const {
    double percentage_of_heap = ((double) entry->words()) / _total_size_in_words;
    return percentage_of_heap >= _size_threshold_percentage;
  }

which requires KlassInfoEntry (defined in src/hotspot/share/memory/heapInspection.hpp). When I tried to change that ifdef to INCLUDE_JFR, building with the `jfr` feature succeeded, but building with `services` failed.

So it seems there is some shared code here. Either heapInspection needs to be included if either `jfr` or `services` are enabled, or we should go the route of this PR and say that JFR is dependent on services. I think the latter seems cleaner.

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PR Comment: https://git.openjdk.org/jdk/pull/26723#issuecomment-3175002573