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

[Johannes Bechberger]

On Wed, 4 Jun 2025 08:40:34 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.cpp line 606:
>> 
>>> 604: void JfrCPUTimeThreadSampler::init_timers() {
>>> 605:   // install sig handler for sig
>>> 606:   PosixSignals::install_generic_signal_handler(SIG, (void*)::handle_timer_signal);
>> 
>> SIGPROF is also used by external profilers. Need to check if SIGPROF handler is already installed and warn user.
>
> This is *very* important to have a robust failure mechanism when existing handlers are already installed. Why? JFR can be turned on dynamically from the outside, at any time, during runtime. A lot of agents could have installed their handlers by then.
> 
> Please describe how you intend to handle the case where someone starts JFR late during runtime and the signal handler cannot be installed.

I added a log_error to tell the user

>> src/hotspot/share/jfr/periodic/sampling/jfrCPUTimeThreadSampler.hpp line 128:
>> 
>>> 126:   static void send_lost_event(const JfrTicks& time, traceid tid, s4 lost_samples);
>>> 127: 
>>> 128:   // Trigger sampling while a thread is not in a safepoint, from a seperate thread
>> 
>> typo: separate
>
> And again, its not "sampling" that is triggered. It is async processing of the queue holding existing samples.

I removed the comment, as the method name itself is pretty self-explanatory.

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PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2126330036
PR Review Comment: https://git.openjdk.org/jdk/pull/25302#discussion_r2126332382