RFR: 8352251: Implement Cooperative JFR Sampling [v16]

[Markus Grönlund]

On Mon, 5 May 2025 20:57:52 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

> > > Isn't this problem already solved by using the `_last_sender_Java_fp`? Why do we need both, the frame pop before the safepoint check and the _last_sender_Java_fp trick?
> 
> > 
> 
> > Because the sampler fetches the CPU context for threads running in state "_thread_in_Java". Without it, you can sample after the safepoint poll test is issued, but before the frame is popped. That sampled frame will represent something now being removed, and if the sender issues another call, also overwritten.
> 
> > 
> 
> > It may help to think about how the JIT methods works - they pop their frames before issuing the method return safepoint poll test.
> 
> 
> 
> Ok. This makes sense. Thanks for the explanation!
> 
> Just trying to understand: What if we have the frame pop before the safepoint check like in JIT compiled code. Do we still need the `_last_sender_Java_fp` trick in this case (assuming we handle it like in the JIT compiled case and use `StackWatermarkSet::after_unwind` in `InterpreterRuntime::at_unwind`)?

Sorry Martin, I did not read your reply detailed enough. That is surely an interesting idea - I did not want to shake things too much, but now I will attempt to try it.

Could solve the trick quite naturally, as you say.

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PR Comment: https://git.openjdk.org/jdk/pull/24296#issuecomment-2853568317