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

[Daniel D. Daugherty]

On Wed, 19 Apr 2023 12:11:58 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 787:
> 
>> 785:   if (jthread != nullptr) {
>> 786:     oop thread_oop = JNIHandles::resolve_non_null(jthread);
>> 787:     if (java_lang_Thread::thread(thread_oop) != nullptr) {
> 
> Suggestion for a comment after this line:
> 
> // We initially have a live JavaThread, so try to capture it in a ThreadsListHandle

I've added:

    if (java_lang_Thread::thread(thread_oop) != nullptr) {
      // Try to capture the live JavaThread in a ThreadsListHandle:
      ThreadsListHandle tlh; // Provides memory barrier

> src/hotspot/share/prims/unsafe.cpp line 789:
> 
>> 787:     if (java_lang_Thread::thread(thread_oop) != nullptr) {
>> 788:       ThreadsListHandle tlh; // Provides memory barrier
>> 789:       if (java_lang_Thread::thread(thread_oop) != nullptr) {
> 
> Suggestion for comment after this line:
> 
> // We still have a live JavaThread, now protected by the ThreadsListHandle.

I've added:

      if (java_lang_Thread::thread(thread_oop) != nullptr) {
        // The still live JavaThread is protected by the ThreadsListHandle.
        JavaThread* thr = nullptr;

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PR Review Comment: https://git.openjdk.org/jdk/pull/13519#discussion_r1171632327
PR Review Comment: https://git.openjdk.org/jdk/pull/13519#discussion_r1171634034