RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v50]

[Aleksey Shipilev]

On Wed, 4 Jun 2025 12:56:22 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:
> 
>   Fix build

Some more cosmetics/nits.

src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 264:

> 262:   }
> 263:   timer_delete(*timer);
> 264:   tl->unset_cpu_timer();

Should this deletion be right in `unset_cpu_timer`?

src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 729:

> 727: #else
> 728: 
> 729: static bool _showed_warning = false;

`_displayed_warning`? Actually, I think you can move this straight into `warn()` body.

src/hotspot/share/jfr/periodic/sampling/jfrThreadSampling.cpp line 250:

> 248:     }
> 249: 
> 250: 

Unnecessary?

src/hotspot/share/jfr/support/jfrThreadLocal.cpp line 582:

> 580: }
> 581: 
> 582: bool JfrThreadLocal::acquire_cpu_time_jfr_enqueue_lock() {

This sounds like `try_acquire_cpu_time_jfr_enqueue_lock`, emphasis on `try_`. It does not actually guarantee to lock.

src/hotspot/share/jfr/support/jfrThreadLocal.cpp line 586:

> 584: }
> 585: 
> 586: bool JfrThreadLocal::try_acquire_cpu_time_jfr_dequeue_lock() {

...and this one is not `try_`, but the actual "blocking" acquire.

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PR Review: https://git.openjdk.org/jdk/pull/25302#pullrequestreview-2896958686
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2126882197
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2126868308
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2126736956
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2126831195
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2126844713