RFR: 8338383: Implementation of Synchronize Virtual Threads without Pinning [v3]

[Alan Bateman]

On Mon, 21 Oct 2024 15:41:45 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Patricio Chilano Mateo has updated the pull request incrementally with six additional commits since the last revision:
>> 
>>  - Fix comments in objectMonitor.hpp
>>  - Move frame::saved_thread_address() to platform dependent files
>>  - Fix typo in jvmtiExport.cpp
>>  - remove usage of frame::metadata_words in possibly_adjust_frame()
>>  - Fix comments in c2 locking paths
>>  - Revert and simplify changes to c1_Runtime1 on aarch64 and riscv
>
> 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).

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1810578179