On Mon, 16 Nov 2020 23:30:25 GMT, Coleen Phillimore <[email protected]> wrote:
>> This change turns the HashTable that JVMTI uses for object tagging into a
>> regular Hotspot hashtable - the one in hashtable.hpp with resizing and
>> rehashing. Instead of pointing directly to oops so that GC has to walk the
>> table to follow oops and then to rehash the table, this table points to
>> WeakHandle. GC walks the backing OopStorages concurrently.
>>
>> The hash function for the table is a hash of the lower 32 bits of the
>> address. A flag is set during GC (gc_notification if in a safepoint, and
>> through a call to JvmtiTagMap::needs_processing()) so that the table is
>> rehashed at the next use.
>>
>> The gc_notification mechanism of weak oop processing is used to notify Jvmti
>> to post ObjectFree events. In concurrent GCs there can be a window of time
>> between weak oop marking where the oop is unmarked, so dead (the phantom
>> load in peek returns NULL) but the gc_notification hasn't been done yet. In
>> this window, a heap walk or GetObjectsWithTags call would not find an object
>> before the ObjectFree event is posted. This is dealt with in two ways:
>>
>> 1. In the Heap walk, there's an unconditional table walk to post events if
>> events are needed to post.
>> 2. For GetObjectWithTags, if a dead oop is found in the table and posting is
>> required, we use the VM thread to post the event.
>>
>> Event posting cannot be done in a JavaThread because the posting needs to be
>> done while holding the table lock, so that the JvmtiEnv state doesn't change
>> before posting is done. ObjectFree callbacks are limited in what they can
>> do as per the JVMTI Specification. The allowed callbacks to the VM already
>> have code to allow NonJava threads.
>>
>> To avoid rehashing, I also tried to use object->identity_hash() but this
>> breaks because entries can be added to the table during heapwalk, where the
>> objects use marking. The starting markWord is saved and restored. Adding a
>> hashcode during this operation makes restoring the former markWord (locked,
>> inflated, etc) too complicated. Plus we don't want all these objects to
>> have hashcodes because locking operations after tagging would have to always
>> use inflated locks.
>>
>> Much of this change is to remove serial weak oop processing for the
>> weakProcessor, ZGC and Shenandoah. The GCs have been stress tested with
>> jvmti code.
>>
>> It has also been tested with tier1-6.
>>
>> Thank you to Stefan, Erik and Kim for their help with this change.
>
> Coleen Phillimore has updated the pull request incrementally with one
> additional commit since the last revision:
>
> Fix minimal build.
Hi Coleen,
It looks good to me.
Just a couple of nits below.
src/hotspot/share/prims/jvmtiTagMap.cpp:
There is a double-check for _needs_cleaning, so the one at line 136 can be
removed:
136 if (_needs_cleaning &&
137 post_events &&
138 env()->is_enabled(JVMTI_EVENT_OBJECT_FREE)) {
139 remove_dead_entries(true /* post_object_free */);
1158 void JvmtiTagMap::remove_dead_entries(bool post_object_free) {
1159 assert(is_locked(), "precondition");
1160 if (_needs_cleaning) {
1161 log_info(jvmti, table)("TagMap table needs cleaning%s",
1162 (post_object_free ? " and posting" : ""));
1163 hashmap()->remove_dead_entries(env(), post_object_free);
1164 _needs_cleaning = false;
1165 }
1166 }
test/hotspot/jtreg/vmTestbase/nsk/jvmti/AttachOnDemand/attach021/attach021Agent00.cpp:
The change below is not needed as the call to
nsk_jvmti_aod_disableEventAndFinish() does exactly the same:
- nsk_jvmti_aod_disableEventAndFinish(agentName, JVMTI_EVENT_OBJECT_FREE,
success, jvmti, jni);
+
+ /* Flush any pending ObjectFree events, which may set success to 1 */
+ if (jvmti->SetEventNotificationMode(JVMTI_DISABLE,
+ JVMTI_EVENT_OBJECT_FREE,
+ NULL) != JVMTI_ERROR_NONE) {
+ success = 0;
+ }
+
+ nsk_aod_agentFinished(jni, agentName, success);
}
-------------
Marked as reviewed by sspitsyn (Reviewer).
PR: https://git.openjdk.java.net/jdk/pull/967
