RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v25]

[Johannes Bechberger]

On Sat, 31 May 2025 10:28:03 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 363:
>> 
>>> 361: }
>>> 362: 
>>> 363: void JfrCPUTimeThreadSampler::stackwalk_thread_in_native(JavaThread* thread) {
>> 
>> I still don't understand what the purpose is for this routine.
>> 
>> I understand that this is to handle the situation where a thread is in state _thread_in_native, and cannot be handled immediately. But what problem are you trying to solve?
>> 
>> It seems there is some convoluted logic to only locate and process those requests that correspond to samples where the thread would be in native? But what purpose does that serve?
>> 
>> In JFR Cooperative Sampling, we allow the sampler thread to drain the entire sample request queue for a thread when it is found to be running in state _thread_in_native, on the premise that we cannot know or guarantee if or when a thread will return to the VM.
>> 
>> This seems to be some kind of semi-solution that does not solve anything:
>> 
>> 1. If you don't care about samples being processed and delivered swiftly, you don't even need the sampler thread to process "native" sample requests - do nothing and let the thread itself process everything on return from native (those native requests are still valid (the ljf is still valid).
>> 2. If you want swift delivery of samples, you drain the entire queue, not just some native samples.
>
> Secondly, if you only want to process native requests that are equal to or correspond to a specific native site, you should only need to call stacktrace.record() inner for one of the sample requests, not all, and reuse the sid for all the requests.

You're right, I'll be just draining the whole queue.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2117831282