On Thu, 29 Jan 2026 04:30:46 GMT, David Holmes <[email protected]> wrote:
>> JDK-8364343 upgraded the virtual thread transition management to be
>> independent of JVMTI. We can update java_lang_Thread::async_get_stack_trace
>> to use it and remove the suspend + retry code from Thread.getStackTrace.
>>
>> A summary of the changes:
>>
>> - java_lang_Thread::async_get_stack_trace is changed to use the new
>> handshake op so it can be called to get the stack trace of a started thread
>> in any state
>> - Thread::getStackTrace is changed to use async_get_stack_trace for all cases
>> - The SUSPENDED substate in VirtualThread is removed
>> - JVM_CreateThreadSnapshot is changed to be usable when JVMTI is not
>> compiled in
>> - ThreadSnapshotFactory::get_thread_snapshot is changed to not upcall to
>> StackTraceElement to complete the init of the stack trace
>>
>> The changes mean that Thread::getStackTrace may be slower when sampling a
>> virtual thread in transition. This case should be rare, and it isn't really
>> a performance critical op anyway. I prototyped use a spin loop and an
>> increasing wait time in MountUnmountDisabler::disable_transition_for_one to
>> avoid the wait(10) but decided to leave it out for now. Future work may
>> examine this issue as there may be other cases (with JVMTI) that would
>> benefit from avoiding the wait.
>>
>> A future PR might propose to change Thread.getStackTrace to use
>> ThreadSnapshot and allow java_lang_Thread::async_get_stack_trace be removed.
>> This requires more extensive changes to ThreadSnapshotFactory to reduce
>> overhead when only the stack trace is required.
>>
>> Testing: tier1-5. The changes has been already been tested in the loom repo
>> for a few months.
>
> src/hotspot/share/services/threadService.cpp line 1479:
>
>> 1477: if (cl._thread_status == JavaThreadStatus::NEW || cl._thread_status
>> == JavaThreadStatus::TERMINATED) {
>> 1478: return nullptr;
>> 1479: }
>
> It is not obvious to me why this is only a possibility now?
ThreadSnapshot.of(Thread) may be invoked with a platform or virtual Thread in
any state. It could sample the thread state before calling
ThreadSnapshotFactory::get_thread_snapshot. That would allow it to filter out
unstarted/NEW threads. It could also filter terminated threads but that would
be racy and get_thread_snapshot would still need to handle terminated threads.
For platform threads, get_thread_snapshot will bail out early if there is no
JavaThread. So the effect of the above is to have virtual threads also be
filtered out.
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PR Review Comment: https://git.openjdk.org/jdk/pull/29461#discussion_r2740654504
