RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v45]

[Johannes Bechberger]

On Wed, 4 Jun 2025 11:21:57 GMT, Andrei Pangin <apangin at openjdk.org> wrote:

>> Johannes Bechberger has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 127 commits:
>> 
>>  - Merge branch 'master' into parttimenerd_cooperative_cpu_time_sampler
>>  - Add error message on signal handler install failure
>>  - Fix signal handler synchronization
>>  - Improve
>>  - Rename autoadapt
>>  - Make process_cpu_time_request private and move up
>>  - Reorder condition
>>  - Tiny refactoring
>>  - Restrict threads for which timers are created
>>  - Fix tiny mistake
>>  - ... and 117 more: https://git.openjdk.org/jdk/compare/7838321b...4fd4b673
>
> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 611:
> 
>> 609:   // increment the count of active signal handlers
>> 610:   u4 old_value = Atomic::fetch_then_add(&_active_signal_handlers, (u4)1, memory_order_acq_rel);
>> 611:   if ((old_value & STOP_SIGNAL_BIT) != 0) {
> 
> Combining stop signal with a counter is nice, you can then use `Atomic::cmpxchg` to avoid incrementing counter when the stop bit is set.

I don't see how `Atomic::cmpxchg` would make the code easier.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2126375482