RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning [v4]

[Patricio Chilano Mateo]

On Tue, 22 Oct 2024 11:51:47 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> src/hotspot/share/runtime/javaThread.cpp line 1545:
>> 
>>> 1543:     if (is_vthread_mounted()) {
>>> 1544:       // _lock_id is the thread ID of the mounted virtual thread
>>> 1545:       st->print_cr("   Carrying virtual thread #" INT64_FORMAT, lock_id());
>> 
>> What is the interaction here with `switchToCarrierThread` and the window between?
>> 
>>         carrier.setCurrentThread(carrier);
>>         Thread.setCurrentLockId(this.threadId());
>> 
>> Will we print the carrier threads id as a virtual threads id? (I am guessing that is_vthread_mounted is true when switchToCarrierThread is called).
>
> Just to say that we hope to eventually remove these "temporary transitions". This PR brings in a change that we've had in the loom repo to not need this when calling out to the scheduler. The only significant remaining use is timed-park. Once we address that then we will remove the need to switch the thread identity and remove some complexity, esp. for JVMTI and serviceability.
> 
> In the mean-time, yes, the JavaThread.lock_id will temporarily switch to the carrier so a thread-dump/safepoint at just the right time looks like it print will be tid of the carrier rather than the mounted virtual thread. So we should fix that. (The original code in main line skipped this case so was lossy when taking a thread dump when hitting this case, David might remember the discussion on that issue).

The problem is that within that window we don't have access to the virtual thread's tid. The current thread has already been changed and we haven't yet set the lock id back. Since this will be a rare corner case maybe we can just print tid unavailable if we hit it. We could also add a boolean to setCurrentThread to indicate we don't want to change the lock_id, but not sure it's worth it.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1811240529