RFR: 8305670: Performance regression in LockSupport.unpark with lots of idle threads

[Robbin Ehn]

On Wed, 19 Apr 2023 12:15:56 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Address the performance regression in `LockSupport.unpark()` with the following changes:
>> - Add a `quick_mode` parameter to `ThreadsListHandle::cv_internal_thread_to_JavaThread()` that tells the conversion function to skip searching the ThreadsList for the target `JavaThread*`.
>> - Update `Unsafe_Unpark()` to verify that `java_lang_Thread::thread(thread_oop) != nullptr` both before and after the creation of the ThreadsListHandle; this verification that a `JavaThread*` is saved in the `java.lang.Thread` object allows us to know that the target `JavaThread*` is protected by the ThreadsListHandle without searching it.
>> - Move the storing of the `JavaThread*` in the `java.lang.Thread` object until after the `JavaThread*` is added to the system ThreadsList.
>> - This change in the order permits the check of `java_lang_Thread::thread(thread_oop) != nullptr` before the creation of the ThreadsListHandle to have the meaning that we need for a `JavaThread*` that is in the early stages of running.
>> - The check `java_lang_Thread::thread(thread_oop) != nullptr` that happens after the creation of the ThreadsListHandle permits the meaning that we need for a `JavaThread*` that is in the end stages of running.
>> - So the combination of the two `java_lang_Thread::thread(thread_oop) != nullptr` checks allows us to safely skip the ThreadsList search in `ThreadsListHandle::cv_internal_thread_to_JavaThread()`.
>> 
>> Additional changes:
>> - Remove the `EnableThreadSMRExtraValidityChecks` option since that option implies that the ThreadsList search is optional in all situations which is not the case. Also remove comments that mention the option.
>> - Update the comments in `ThreadsListHandle::cv_internal_thread_to_JavaThread()` to clarify what `java_lang_Thread::thread(thread_oop) != nullptr` means in the JavaThread's lifecycle and that function's attempts to convert a `jthread` into a `JavaThread*` that's known to be protected by the ThreadsListHandle.
>
> src/hotspot/share/prims/unsafe.cpp line 794:
> 
>> 792:         // before and after creating tlh above so quick_mode can be used in
>> 793:         // this conversion call.
>> 794:         if (tlh.cv_internal_thread_to_JavaThread(jthread, &thr, nullptr, true /* quick_mode */)) {
> 
> Sorry I'm having a mental blank here now. Why do we even need to call this if we can access `java_lang_Thread::thread(thread_oop)` after the TLH was created?

As a caller of cv_internal_thread_to_JavaThread we do not know the implementation of quick_mode.

As the implementation is now, we still want the asserts.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13519#discussion_r1171578848