RFR: 8342818: Implement JEP 509: JFR CPU-Time Profiling [v10]

[Johannes Bechberger]

On Wed, 4 Jun 2025 04:50:56 GMT, David Holmes <dholmes at openjdk.org> wrote:

>>> I wonder if <Field type="Thread" name="eventThread" label="Thread" /> is needed, instead of thread = true?
>> 
>> We had these discussions before on the old PR and then decided to end up with eventThread (as the other events do to),
>
> @parttimenerd  I would really like to see some kind of design description for this which explains what the threading model is, how the signals are used, and how all the pieces interact. Thanks

@dholmes-ora I attempt a first version here:

The design consists of four main parts:
- setup code: This sets up the signal handlers for every new thread and deletes them afterwards
- the per-thread signal handlers: They check first that the current thread is valid, increment that they are currently active and check that they shouldn't stop (because the profiler is disabled). Now they acquire the thread-local enqueue lock for the current thread's request queue and push the sampling requests in (see https://openjdk.org/jeps/518 + the current period). It triggers/arms a safepoint. If the current thread is in native, they trigger (set a flag) the asynchronous stackwalking. This prevents long native periods of overflowing the request queue. Finally, the enqueue lock is released.
- the safepoint handler: In the safepoint handler, we check if the thread-local queue is not empty. If so, we acquire a dequeue lock and process all entries of the queue, thereby creating JFR events. We also untrigger the async-stack-walking request for the thread. We then release the lock.
- the sampler thread: Its task is to regularly update the timers if needed (configuration changes) and to walk the thread list to find any task that wants to be asynchronously stack-walked. For every of these threads, the dequeue lock is acquired (skipping if already set to enqueue) and the queue is processed as at the safepoint. Then the lock is released.

On shutdown: Whenever the sampler is shut down, we first set the `_stop_signals` flag to prevent new signal handlers from entering the request creation code (and thereby accessing data structures that we already deallocated), we disable the timers for all threads and then wait till no signal handler is engaged anymore.

It is important to note that there is only one thread-local lock used, but it has three states:
- enqueue
- dequeue
- unlocked

This prevents these phases from overlapping.

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PR Comment: https://git.openjdk.org/jdk/pull/25302#issuecomment-2938677600