RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v55]

[Aleksey Shipilev]

On Wed, 4 Jun 2025 15:48:31 GMT, Johannes Bechberger <jbechberger at openjdk.org> wrote:

>> This is the code for the [JEP 509: CPU Time based profiling for JFR](https://openjdk.org/jeps/509).
>> 
>> Currently tested using [this test suite](https://github.com/parttimenerd/basic-profiler-tests). This runs profiles the [Renaissance](https://renaissance.dev/) benchmark with
>> - ... different heap sizes
>> - ... different GCs
>> - ... different samplers (the standard JFR and the new CPU Time Sampler and both)
>> - ... different JFR recording durations
>> - ... different chunk-sizes
>
> Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve

I admit I am leaning pretty hard on JFR folks expertise here.

I agree this code passes the bar for experimental feature: it does not seem to affect non-JFR paths, does not seem to interact with JFR in obviously incorrect manner, and in itself looks more or less sensible.

Please file the follow-ups to figure out the memory ordering story in `JfrCPUTimeTraceQueue`.

src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 640:

> 638: 
> 639:   VM_CPUTimeSamplerThreadInitializer(JfrCPUSamplerThread* sampler) : _sampler(sampler) {
> 640:   }

Suggestion:

 private:
  JfrCPUSamplerThread* const _sampler;
  
 public:
  VM_CPUTimeSamplerThreadInitializer(JfrCPUSamplerThread* sampler) : _sampler(sampler) {}

src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 671:

> 669: 
> 670:   VM_CPUTimeSamplerThreadTerminator(JfrCPUSamplerThread* sampler) : _sampler(sampler) {
> 671:   }

Suggestion:

 private:
  JfrCPUSamplerThread* const _sampler;

 public:
  VM_CPUTimeSamplerThreadTerminator(JfrCPUSamplerThread* sampler) : _sampler(sampler) {}

src/hotspot/share/runtime/vmOperation.hpp line 119:

> 117:   template(RendezvousGCThreads)                   \
> 118:   template(CPUTimeSamplerThreadInitializer)       \
> 119:   template(CPUTimeSamplerThreadTerminator)        \

I think these better be prefixed with `JFR`. E.g.: `JFRInitializeCPUTimeSampler` / `JFRTerminateCPUTimeSampler`?

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

Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25302#pullrequestreview-2897511962
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2127078325
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2127079549
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2127090498