RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v42]

[Johannes Bechberger]

On Wed, 4 Jun 2025 00:13:07 GMT, Andrei Pangin <apangin at openjdk.org> wrote:

>> Johannes Bechberger has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Rename autoadapt
>
> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 161:
> 
>> 159:     return 0;
>> 160:   }
>> 161:   return os::active_processor_count() * 1000000000.0 / rate;
> 
> If sampling period is configured as an absolute number in milliseconds, this value must be passed as is.
> Double conversion via `Runtime.availableProcessors()` / `active_processor_count()` is unobvious and error-prone. First, because of asymmetry: e.g. `Runtime.availableProcessors()` may be redefined by an agent so that its value is not aligned with `active_processor_count()`. Second, because number of available processors may change at runtime, e.g., by adjusting cgroup quotas.

Is this something for a later PR?

> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 281:
> 
>> 279:     stop_timer();
>> 280:     Atomic::store(&_stop_signals, true);
>> 281:     while (Atomic::load_acquire(&_active_signal_handlers) > 0) {
> 
> There can be a race when `handle_timer_signal` has already passed `_stop_signals` check but has not yet incremented `_active_signal_handlers`.

Amy idea on how to fix it?

> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 472:
> 
>> 470: 
>> 471: void handle_timer_signal(int signo, siginfo_t* info, void* context) {
>> 472:   assert(_instance != nullptr, "invariant");
> 
> There can be an arbitrary delay in async signal delivery.
> It's unlikely, but not impossible for `_instance` to be deleted by the time signal handler is called. There should be a better way to synchronize with JFR shutdown.

Any ideas? Or is it something for a later PR?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2125678084
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2125680345
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2125681876