RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v5]

[Johannes Bechberger]

On Sun, 25 May 2025 10:16:29 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>> Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix compilation
>
> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 200:
> 
>> 198:   void sample_thread(JfrSampleRequest& request, void* ucontext, JavaThread* jt, JfrThreadLocal* tl);
>> 199: 
>> 200:   // sample all marked threads out of safepoint
> 
> What is meant by "out of safepoint"?

Stack walking that occurs not at a safepoint, but during longer periods of a thread being in native. We can't walk the stack directly when we're in native, as we can't properly walk stacks in signal handlers. So we defer this to a separate thread.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2106647626