RFR: 8337789: JEP 509: JFR CPU-Time Profiling (Experimental) [v47]

[Zhengyu Gu]

On Mon, 5 May 2025 14:22:07 GMT, Johannes Bechberger <jbechberger at openjdk.org> wrote:

>> This is the code for the [JEP draft: CPU Time based profiling for JFR].
>> 
>> Currently tested using [this test suite](https://github.com/parttimenerd/basic-profiler-tests).
>> 
>> A version based on the cooperative sampling JEP can be found [here](https://github.com/parttimenerd/jdk/tree/parttimenerd_cooperative_cpu_time_sampler).
>
> Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Simplify local trace stack

src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 331:

> 329:       u4 state = Atomic::load_acquire(&e->_state);
> 330:       if (state == state_empty(tail)) {
> 331:         if (Atomic::cmpxchg(&_tail, tail, tail + 1, memory_order_seq_cst) == tail) {

I think it still has race. 

For example, _tail = 0, _capacity = 2
T1 enqueue: claim position 0,  _tail = 1 after CAS
T2 enqueue: claim position 1,  _tail = 2 after CAS
T2 enqueue: store value at position 1
T2 enqueue: claim position 0, _tail = 3 after CAS
T2 enqueue: store value at position 0
T1 enqueue: store value at position 0

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PR Review Comment: https://git.openjdk.org/jdk/pull/20752#discussion_r2076150740