Hi Coleen,
Sorry for the delay in getting back to this re-review.
On 11/21/19 9:12 AM, [email protected] wrote:
Please review a new version of this change that keeps the nmethod from
being unloaded, after it is added to the deferred event queue:
http://cr.openjdk.java.net/~coleenp/2019/8173361.03/webrev/index.html
src/hotspot/share/code/nmethod.cpp
No comments.
src/hotspot/share/oops/instanceKlass.cpp
No comments.
src/hotspot/share/prims/jvmtiExport.cpp
No comments.
src/hotspot/share/prims/jvmtiImpl.cpp
Nice solution with the new oops_do() and nmethods_do() functions!
old L988: void JvmtiDeferredEventQueue::enqueue(const
JvmtiDeferredEvent& event) {
new L998: void JvmtiDeferredEventQueue::enqueue(JvmtiDeferredEvent
event) {
Not sure why this was changed.
Update: Looks like Serguei raised the issue and Coleen has already
resolved it.
src/hotspot/share/prims/jvmtiImpl.hpp
old L494: QueueNode(const JvmtiDeferredEvent& event)
new L498: QueueNode(JvmtiDeferredEvent& event)
Why was this changed?
Update: Not clear if this was covered by Coleen's reply to Serguei.
old L497: const JvmtiDeferredEvent& event() const { return
_event; }
new L501: JvmtiDeferredEvent& event() { return _event; }
Why was this changed?
Update: Coleen's reply to Serguei explained this. Perhaps add:
// Not const because of oops_do() and nmethods_do().
old L509: static void enqueue(const JvmtiDeferredEvent& event)
NOT_JVMTI_RETURN;
new L513: static void enqueue(JvmtiDeferredEvent event)
NOT_JVMTI_RETURN;
Why was this changed?
Update: Looks like Serguei raised the issue and Coleen has already
resolved it.
src/hotspot/share/runtime/mutexLocker.cpp
This change is going to require some testing to make sure we don't
have any new deadlock scenarios.
src/hotspot/share/runtime/serviceThread.cpp
L50 - nit - why the extra blank line?
src/hotspot/share/runtime/serviceThread.hpp
Thanks for cleaning up the static:
ServiceThread::is_service_thread(Thread* thread)
stuff. Having it be different than the other threads was
a bit jarring.
src/hotspot/share/runtime/thread.hpp
No comments.
Thumbs up. My only comments are nits so I don't need to see a
new webrev if you decide to fix them.
Dan
Ran the test that failed 100 times without failure, tier1 on Oracle
supported platforms, and tier2-3 including jvmti and jdi tests locally.
See bug for more details about the crash.
https://bugs.openjdk.java.net/browse/JDK-8173361
Thanks,
Coleen
On 11/18/19 10:09 PM, [email protected] wrote:
Hi Serguei,
Sorry for not sending an update. I talked to Erik and am working on
a version that keeps the nmethod from being unloaded while it's in
the deferred event queue, with a version that the GC people will
like, and I like. I'm testing it out now.
Thanks!
Coleen
On 11/18/19 10:03 PM, [email protected] wrote:
Hi Coleen,
Sorry for the latency, I had to investigate it a little bit.
I still have some doubt your fix is right thing to do.
On 11/16/19 04:55, [email protected] wrote:
On 11/15/19 11:17 PM, [email protected] wrote:
Hi Coleen,
On 11/15/19 2:12 PM, [email protected] wrote:
Hi, I've been working on answers to these questions, so I'll
start with this one.
The nmethodLocker keeps the nmethod from being reclaimed
(made_zombie or memory released) by the sweeper, but the nmethod
could be unloaded. Unloading the nmethod clears the Method*
_method field.
Yes, I see it is done in the nmethod::make_unloaded().
The post_compiled_method_load event needs the _method field to
look at things like inlining and ScopeDesc fields. If the
nmethod is unloaded, some of the oops are dead. There are
"holder" oops that correspond to the metadata in the nmethod. If
these oops are dead, causing the nmethod to get unloaded, then
the metadata may not be valid.
So my change 02 looks for a NULL nmethod._method field to tell
whether we can post information about the nmethod.
There's code in nmethod.cpp like:
jmethodID nmethod::get_and_cache_jmethod_id() {
if (_jmethod_id == NULL) {
// Cache the jmethod_id since it can no longer be looked up
once the
// method itself has been marked for unloading.
_jmethod_id = method()->jmethod_id();
}
return _jmethod_id;
}
Which was added when post_method_load and unload were turned into
deferred events.
Could we cache the jmethodID in the
JvmtiDeferredEvent::compiled_method_load_event
similarly as we do in the
JvmtiDeferredEvent::compiled_method_unload_event?
This would help to get rid of the dependency on the nmethod::_method.
Do we depend on any other nmethod fields?
Yes, there are other nmethod metadata that we rely on to print
inline information, and this function
JvmtiCodeBlobEvents::build_jvmti_addr_location_map because it uses
the ScopeDesc data in the nmethod.
One possible approach is to prepare and cache all this information
in the nmethod::post_compiled_method_load_event() before the
JvmtiDeferredEvent::compiled_method_load_event() is called.
The event parameters are:
typedef struct {
const void* start_address;
jlocation location;
} jvmtiAddrLocationMap;
CompiledMethodLoad(jvmtiEnv *jvmti_env,
jmethodID method,
jint code_size,
const void* code_addr,
jint map_length,
const jvmtiAddrLocationMap* map,
const void* compile_info)
Some of these addresses above could be not accessible when an event
is posted.
Not sure yet if it is Okay.
The question is if this kind of refactoring is worth and right thing
to do.
We do cache the jmethodID but that's not good enough. See my last
comment in the bug report. The jmethodID can point to an unloaded
method.
This looks like it is done a little bit late.
It'd better to do it before the event is deferred (see above).
I tried a version of keeping the nmethod alive, but the GC folks
will hate it. And it doesn't work and I hate it.
From serviceability point of view this is the best and most
consistent approach.
I seems to me, it was initially designed this way.
The downside is it adds some extra complexity to the GC.
My version 01 is the best, with the caveat that maybe it should
check for _method == NULL instead of nmethod->is_alive(). I have
to talk to Erik to see if there's a race with concurrent class
unloading.
Any application that depends on a compiled method loading event on
a class that could be unloaded is a buggy application.
Applications should not rely on when the JIT compiler decides to
compile a method! This happens to us for a stress test. Most
applications will get most of their compiled method loading events
as they normally do.
It is not an application that relies on the compiled method loading
event.
It is about profiling tools to be able to get correct information
about what is going on with compilations.
My concern is that if we skip such compiled method load events then
profilers have no way
to find out there many unneeded compilations that are thrown away
without any real use.
Also, it is not clear what happens with the subsequent compiled
method unload events.
Are they going to be skipped as well or they can appear and confuse
profilers?
Thanks,
Serguei
Thanks,
Coleen
Thanks,
Serguei
I put more debugging in the bug to show this crash was from an
unloaded nmethod.
Coleen
On 11/15/19 4:45 PM, [email protected] wrote:
Hi Coleen,
I have some questions.
Both the compiler method load and unload are posted as deferred
events.
Both events keep the nmethod alive until the ServiceThread
processes the event.
The implementation is:
JvmtiDeferredEvent
JvmtiDeferredEvent::compiled_method_load_event(nmethod* nm) {
. . .
// Keep the nmethod alive until the ServiceThread can process
// this deferred event.
nmethodLocker::lock_nmethod(nm);
return event;
}
JvmtiDeferredEvent
JvmtiDeferredEvent::compiled_method_unload_event(nmethod* nm,
jmethodID id, const void* code) {
. . .
// Keep the nmethod alive until the ServiceThread can process
// this deferred event. This will keep the memory for the
// generated code from being reused too early. We pass
// zombie_ok == true here so that our nmethod that was just
// made into a zombie can be locked.
nmethodLocker::lock_nmethod(nm, true /* zombie_ok */);
return event;
}
void JvmtiDeferredEvent::post() {
assert(ServiceThread::is_service_thread(Thread::current()),
"Service thread must post enqueued events");
switch(_type) {
case TYPE_COMPILED_METHOD_LOAD: {
nmethod* nm = _event_data.compiled_method_load;
JvmtiExport::post_compiled_method_load(nm);
// done with the deferred event so unlock the nmethod
nmethodLocker::unlock_nmethod(nm);
break;
}
case TYPE_COMPILED_METHOD_UNLOAD: {
nmethod* nm = _event_data.compiled_method_unload.nm;
JvmtiExport::post_compiled_method_unload(
_event_data.compiled_method_unload.method_id,
_event_data.compiled_method_unload.code_begin);
// done with the deferred event so unlock the nmethod
nmethodLocker::unlock_nmethod(nm);
break;
}
. . .
}
}
Then I wonder how is it possible for the nmethod to be not alive
here?:
2168 void JvmtiExport::post_compiled_method_load(nmethod *nm) {
. . .
2173 // It's not safe to look at metadata for unloaded methods.
2174 if (!nm->is_alive()) {
2175 return;
2176 }
At least, it lokks like something else is broken.
Do I miss something important here?
Thanks,
Serguei
On 11/14/19 5:15 PM, [email protected] wrote:
Summary: Don't post information which uses metadata from
unloaded nmethods
Tested tier1-3 and 100 times with test that failed (reproduced
failure without the fix).
open webrev at
http://cr.openjdk.java.net/~coleenp/2019/8173361.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8173361
Thanks,
Coleen
