RFR: 8366847: JFR reports incorrect number of cores on hybrid CPU [v3]

[Yasumasa Suenaga]

On Fri, 3 Oct 2025 06:55:56 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Yasumasa Suenaga has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:
>> 
>>  - Merge branch 'master' into jfr-on-hy-cpu
>>  - Use jmc_undefined_long if runs on hybrid CPU
>>  - Revert "Update condition"
>>    
>>    This reverts commit ebf4aec23fa95c8d54d1196c7be85e99f0846420.
>>  - Update condition
>>  - Add fallback code if logical_cpus == 0
>>  - 8365633: JFR reports incorrect number of cores on hybrid CPU
>
> src/hotspot/cpu/x86/vm_version_x86.cpp line 2595:
> 
>> 2593:   // estimate the number of cores.
>> 2594:   // -1 if hybrid CPU because it is difficult to derive number of cores.
>> 2595:   _no_of_cores = supports_hybrid() ? -1 : (cores_per_cpu() * _no_of_sockets);
> 
> It is only meant to be an estimate and -1 is not an estimate at all it is a "I don't know" answer. But why do we not know - using my earlier example won't number of cores be 8 as expected 6P+2E? Even if not it would yield a better approximation than -1.

I think incorrect value is not useful because it causes confusing.

If we have to keep current code to set estimated value, I can use `VM_Version::supports_hybrid()` in JFR code to emit -1 on the event.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27080#discussion_r2401819924