RFR: 8299414: JVMTI FollowReferences should support references from VirtualThread stack [v14]

[Serguei Spitsyn]

On Thu, 4 May 2023 23:20:21 GMT, Alex Menkov <amenkov at openjdk.org> wrote:

>> The fix updates JVMTI FollowReferences implementation to report references from virtual threads:
>> - unmounted vthreads are detected, their stack references for JVMTI_HEAP_REFERENCE_STACK_LOCAL/JVMTI_HEAP_REFERENCE_JNI_LOCAL;
>> - stacks of mounted vthreads are splitted into 2 parts (virtual thread stack and carrier thread stack), references are reported with correct thread id/class tag/object tags/frame depth;
>> - common code to handle stack frames are moved into separate class;
>> 
>> Threads are reported as:
>> - platform threads: JVMTI_HEAP_REFERENCE_THREAD (as before);
>> - mounted vthreads (synthetic references, consider them as heap roots because carrier threads are roots): JVMTI_HEAP_REFERENCE_OTHER;
>> - unmounted vthreads: not reported as heap roots.
>
> Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Updated test

test/hotspot/jtreg/serviceability/jvmti/vthread/FollowReferences/libVThreadStackRefTest.cpp line 106:

> 104: extern "C" JNIEXPORT jint JNICALL
> 105: Agent_OnLoad(JavaVM *vm, char *options, void *reserved) {
> 106:   if (vm->GetEnv(reinterpret_cast<void **>(&jvmti), JVMTI_VERSION) != JNI_OK || jvmti == nullptr) {

Nit: This line is long and non readable. There are many examples in tests how it is normally done.

test/hotspot/jtreg/serviceability/jvmti/vthread/FollowReferences/libVThreadStackRefTest.cpp line 113:

> 111:   memset(&capabilities, 0, sizeof(capabilities));
> 112:   capabilities.can_tag_objects = 1;
> 113:   //capabilities.can_support_virtual_threads = 1;

The line 113 can be removed.

test/hotspot/jtreg/serviceability/jvmti/vthread/FollowReferences/libVThreadStackRefTest.cpp line 130:

> 128: Java_VThreadStackRefTest_test(JNIEnv* env, jclass clazz, jobjectArray classes) {
> 129:   jsize classesCount = env->GetArrayLength(classes);
> 130:   for (int i=0; i<classesCount; i++) {

Spaces are missed arounf '=' and '<' signs.

test/hotspot/jtreg/serviceability/jvmti/vthread/FollowReferences/libVThreadStackRefTest.cpp line 154:

> 152: }
> 153: 
> 154: static void printtCreatedClass(JNIEnv* env, jclass cls) {

Why is printt with 'tt' ?

test/hotspot/jtreg/serviceability/jvmti/vthread/FollowReferences/libVThreadStackRefTest.cpp line 167:

> 165: 
> 166: extern "C" JNIEXPORT void JNICALL
> 167: Java_VThreadStackRefTest_createObjAndCallback(JNIEnv* env, jclass clazz, jclass cls, jobject callback) {

Some comment would be helpful about what this function does.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1185720838
PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1185720066
PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1185721404
PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1185722065
PR Review Comment: https://git.openjdk.org/jdk/pull/13254#discussion_r1185722636