Re: RFR: 8242428: JVMTI thread operations should use Thread-Local Handshake

Wed, 08 Jul 2020 09:44:22 -0700 

> http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.08/

src/hotspot/share/prims/jvmtiEnv.cpp
    No comments.

src/hotspot/share/prims/jvmtiEnvBase.cpp
    L1159:   Thread *current_thread = Thread::current();
        Please add "#ifdef ASSERT" above and "#endif" below since
        current_thread is only used for the assert() in this function.

src/hotspot/share/prims/jvmtiEnvBase.hpp
    L549:                              jthread java_thread, jint max_frame_count) 
    L552:       _jthread(java_thread),
        Please: s/java_thread/thread/ on both lines.

src/hotspot/share/runtime/vmOperations.hpp
    No comments.

test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/OneGetThreadListStackTraces.java
    No comments.

test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libOneGetThreadListStackTraces.cpp
    L27: #include <errno.h>
        This include is out of order; should be first in the list.

    This file doesn't compile on my MBP13:
./open/test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libOneGetThreadListStackTraces.cpp:49:14: error: format specifies type 'unsigned long' but the argument has type 'jmethodID' (aka '_jmethodID *') [-Werror,-Wformat] 
             fi1->method, fi2->method);
             ^~~~~~~~~~~
./open/test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libOneGetThreadListStackTraces.cpp:49:27: error: format specifies type 'unsigned long' but the argument has type 'jmethodID' (aka '_jmethodID *') [-Werror,-Wformat] 
             fi1->method, fi2->method);
                          ^~~~~~~~~~~
    2 errors generated.

    This change made it compile on my MBP13, but that may break it on
    other platforms:

    $ hg diff
    diff -r 560847c69fbe test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libOneGetThreadListStackTraces.cpp     --- a/test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libOneGetThreadListStackTraces.cpp Wed Jul 08 12:13:32 2020 -0400     +++ b/test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libOneGetThreadListStackTraces.cpp Wed Jul 08 12:40:42 2020 -0400 
    @@ -46,7 +46,7 @@
       if (fi1->method != fi2->method) { /* jvmtiFrameInfo::method */
         snprintf(err_msg, sizeof(err_msg),
                  "method is different: fi1 = %lx, fi2 = %lx",
    -             fi1->method, fi2->method);
    +             (unsigned long) fi1->method, (unsigned long) fi2->method); 
         env->FatalError(err_msg);
       } else if (fi1->location != fi2->location) { /* jvmtiFrameInfo::location */ 
         snprintf(err_msg, sizeof(err_msg),

    I'm not sure of the right platform independent way to output
    the 'method' field.

Dan


On 7/8/20 4:04 AM, Yasumasa Suenaga wrote:

Hi David,

On 2020/07/08 15:27, David Holmes wrote:

Hi Yasumasa,

On 7/07/2020 6:54 pm, Yasumasa Suenaga wrote:

Hi David, Serguei,

Serguei, thank you for replying even though you are on vacaiton!

I uploaded new webrev:

   http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.07/
   Diff from previous webrev: http://hg.openjdk.java.net/jdk/submit/rev/77243b1dcbfe
c'tor of GetSingleStackTraceClosure has jthread argument in this webrev. Also it does not contain testcase for GetThreadListStackTraces with all threads, and OneGetThreadListStackTraces would test main thread only. 

All those changes are fine in principle for me. One nit/suggestion:

src/hotspot/share/prims/jvmtiEnvBase.hpp

  544   jthread _java_thread;
elsewhere "java_thread" refers to a JavaThread, so to avoid confusion may I suggest this member be named _jthread. 

I uploaded new webrev:

  http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.08/
  Diff from previous webrev: http://hg.openjdk.java.net/jdk/submit/rev/ca6263dbdc87
I'm going to be away for the next couple of days - sorry - but will try to check email on this if I can. 

Thanks!


Yasumasa



Thanks,
David
-----



Thanks,

Yasumasa


On 2020/07/07 15:13, David Holmes wrote:

On 7/07/2020 2:57 pm, Yasumasa Suenaga wrote:

Hi David,

On 2020/07/07 11:31, David Holmes wrote:

Hi Yasumasa,
Hard to keep up with the changes - especially without incremental webrevs. 

Sorry, I will upload diff from previous webrev in the next.
If GetSingleStackTraceClosure also took the jthread as a constructor arg, then you wouldn't need to recreate a JNI local handle when calling _collector.fill_frames. It's a small simplification and not essential at this stage.
I think we should get jthread from an argument of do_thread() because do_thread() would pass the thread which are stopped certainly. It might be simplification if we pass _calling_thread to MultipleStackTracesCollector. `jthread` is only needed to store jvmtiStackInfo.thread . What do you think? 

I'm not quite sure what you mean.
I think there is a bit of a design wart with direct handshakes in that do_thread takes the target JavaThread as an argument. That's useful in a case where you want a HandshakeClosure that can be applied to multiple threads, but that's not typically what is needed with direct handshakes - there is only a single target. With a single-target HandshakeClosure you can capture all the "target" information for the operation in the closure instance. So if the actual do_thread operation wants the jthread corresponding to the target thread then we can store that in the closure rather than recomputing it (you could assert it is the same but that seems overkill to me).
For the test ... I don't see how Java_GetThreadListStackTraces_checkCallStacks is a valid test. It gets the stacks of all live threads, then uses that information to use GetThreadListStackTraces to get the stack for the same set of threads through a different API. It then compares the two sets of stacks for each thread expecting them to be the same, but that need only be the case for the main thread. Other threads could potentially have a different stack (e.g. if this test is run with JFR enabled there will be additional threads found.) Further I would have expected that there already exist tests that check that, for a given thread (which may be suspended or known to be blocked) the same stack is found through the two different APIs.
vmTestbase/nsk/jvmti/unit/GetAllStackTraces/getallstktr001 would check all of threads via GetThreadListStackTraces() and GetAllStackTraces(), so we might be able to remove GetThreadListStackTraces.java from this webrev.
Yes. The existing test only examines a set of test threads that are all blocked on a raw monitor. You do not need to duplicate that test.
OTOH we don't have testcase for GetThreadListStackTraces() with thread_count == 1, so we need to testcase for it (it is OneGetThreadListStackTraces.java) It would check whether the state of target thread is "waiting" before JNI call to call GetThreadListStackTraces(),
Yes we need to test the special cases introduced by your changes - totally agree - and OneGetThreadListStackTraces.java is a good test for that.
and also I expect it would not be run with JFR. (it is not described @run)
The arguments to run with JFR (or a bunch of other things) can be passed to the jtreg test harness to be applied to all tests.
Of course we can check GetThreadListStackTraces() with main thread, but it is not the test for direct handshake for other thread. 

Right - that test already exists as per the above.

Thanks,
David



Thanks,

Yasumasa



Thanks,
David
-----

On 6/07/2020 11:29 pm, Yasumasa Suenaga wrote:

Hi Serguei,

Thanks for your comment!

I think C++ is more simple to implement the test agent as you said.
So I implement it in C++ in new webrev. Could you review again?

http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.06/
Also I refactored libGetThreadListStackTraces.cpp, and I've kept exception check after IsSameObject(). 


Thanks,

Yasumasa


On 2020/07/06 16:32, serguei.spit...@oracle.com wrote:

Hi Yasumasa,

Thank you for the update.
I think, a pending exception after IsSameObject needs to be checked.
The checkStackInfo() needs one more refactoring as I've already suggested. The body of the loop at L68-L78 should be converted to a function check_frame_info. The si1->frame_buffer[i] and si2->frame_buffer[i] will be passed as fi1 and fi2. 
The index can be passed as well.
I'm still suggesting to simplify the local exception_msg to something shorter like err_msg or exc_msg. 

I'm not sure using fatal is right here:

This fragment looks strange:
  152     if ((*env)->IsSameObject(env, stack_info[i].thread, thread)) { 
  153       target_info = &stack_info[i];
  154       break;
  155     } else if ((*env)->ExceptionOccurred(env)) {
  156       (*env)->ExceptionDescribe(env);
  157       (*env)->FatalError(env, __FILE__);
  158     }

I expected it to be:
    jboolean same = (*env)->IsSameObject(env, stack_info[i].thread, thread); 
    if ((*env)->ExceptionOccurred(env)) {
      (*env)->ExceptionDescribe(env);
      (*env)->FatalError(env, __FILE__);
    }
    if (same) {
      target_info = &stack_info[i];
      break;
    }
Would it better to port this agent to C++ to simplify this code nicer? 

Thanks,
Serguei


On 7/5/20 06:13, Yasumasa Suenaga wrote:

Hi Serguei,

Thanks for your comment!
I refactored testcase. Could you review again?

http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.05/
It would check Java exception after IsSameObject() call. Does it need? Any exceptions are not described in JNI document[1], and JNI implementation (jni_IsSameObject()) does not seem to throw it. 


Thanks,

Yasumasa
[1] https://docs.oracle.com/en/java/javase/14/docs/specs/jni/functions.html#issameobject 


On 2020/07/05 14:46, serguei.spit...@oracle.com wrote:

Hi Yasumasa,


Okay, thanks.
Then I'm okay to keep the GetSingleStackTraceClosure.
http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.04/test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libGetThreadListStackTraces.c.html http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.04/test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libOneGetThreadListStackTraces.c.html 

I'm not sure the function 'is_same_thread() is needed.
Why do not use the JNI IsSameObject instead?

It seems to be a typo at L132 and L137.
You, probably. did not want to print the same information for stack_info_1[i].frame_buffer[j].XXX twice. 

The code at lines 112-142 is not readable.
I'd suggest to make a couple of refactoring steps.
First step to simplify this a little bit would be with some renaming and getting rid of indexes: 

   71 char err_msg[EXCEPTION_MSG_LEN] = {0};
  ...
  112   /* Iterate all jvmtiStackInfo to check */
  113   for (i = 0; i < num_threads, *exception_msg != '\0'; i++) { 
          jvmtiStackInfo *si1 = stack_info_1[i];
          jvmtiStackInfo *si2 = stack_info_2[i];
  114     if (!IsSameObject(env, si1.thread, si2.thread)) { /* jvmtiStackInfo::thread */ 
  115       snprintf(err_msg, sizeof(err_msg),
  116                "thread[%d] is different: stack_info_1 = %p, stack_info_2 = %p", 
  117                i, sinfo1.thread, sinfo2.thread);
  118     } else if (si1.state != si2.state) { /* jvmtiStackInfo::state */ 
  119       snprintf(err_msg, sizeof(err_msg),
  120                "state[%d] is different: stack_info_1 = %d, stack_info_2 = %d", 
  121                i, si1.state, si2.state);
  122     } else if (si1.frame_count != si2.frame_count) { /* jvmtiStackInfo::frame_count */ 
  123       snprintf(err_msg, sizeof(err_msg),
  124                "frame_count[%d] is different: stack_info_1 = %d, stack_info_2 = %d", 
  125                i, si1.frame_count, si2.frame_count);
  126     } else {
  127       /* Iterate all jvmtiFrameInfo to check */
  128       for (j = 0; j < si1.frame_count; j++) {
  129         if (si1.frame_buffer[j].method != si1.frame_buffer[j].method) { /* jvmtiFrameInfo::method */ 
  130           snprintf(err_msg, sizeof(err_msg),
  131                    "thread [%d] frame_buffer[%d].method is different: stack_info_1 = %lx, stack_info_2 = %lx",   132                    i, j, si1.frame_buffer[j].method, si2.frame_buffer[j].method); 
  133           break;
  134         } else if (si1.frame_buffer[j].location != si1.frame_buffer[j].location) { /* jvmtiFrameInfo::location */ 
  135           snprintf(err_msg, sizeof(err_msg),
  136                    "thread [%d] frame_buffer[%d].location is different: stack_info_1 = %ld, stack_info_2 = %ld",   137                    i, j, si1.frame_buffer[j].location, si2.frame_buffer[j].location); 
  138           break;
  139         }
  140       }
  141     }
  142   }
Another step would be to create functions that implement a body of each loop. You can use the same techniques to simplify similar place (L127-L138) in the libOneGetThreadListStackTraces.c. 

Thanks,
Serguei


On 7/3/20 15:55, Yasumasa Suenaga wrote:

Hi Serguei,
I'm not an Oracle employee, so I cannot know real request(s) from your customers. However JDK-8201641 says Dynatrace has requested this enhancement. 

BTW I haven't heared any request from my customers about this.


Thanks,

Yasumasa


On 2020/07/04 4:32, serguei.spit...@oracle.com wrote:

Hi Yasumasa,

This difference is not that big to care about.
I feel this is really rare case and so, does not worth these complications. 
Do we have a real request from customers to optimize it?

Thanks,
Serguei


On 7/3/20 01:16, Yasumasa Suenaga wrote:

Hi Serguei,
Generally I agree with you, but I have concern about the difference of the result of GetStackTrace() and GetThreadListStackTraces(). 

  GetStackTrace: jvmtiFrameInfo
  GetThreadListStackTraces: jvmtiStackInfo
jvmtiStackInfo contains thread state, and it is ensured it is the state of the call stack. If we want to get both call stack and thread state, we need to suspend target thread, and call both GetStackTrace() and GetThreadState(). Is it ok?
I was wondering if JDK-8201641 (parent ticket of this change) needed them for profiling (dynatrace?) If it is responsibility of JVMTI agent implementor, I remove this closure. 


Thanks,

Yasumasa


On 2020/07/03 16:45, serguei.spit...@oracle.com wrote:

Hi Yasumasa,
After some thinking I've concluded that I do not like this optimization of the GetThreadListStackTraces with GetSingleStackTraceClosure. 

We may need more opinions on this but these are my points:
  - it adds some complexity and ugliness
  - a win is doubtful because it has to be a rare case, so that total overhead should not be high   - if it is really high for some use cases then it is up to the user 
    to optimize it with using GetStackTrace instead
In such cases with doubtful overhead I usually prefer the simplicity.
Good examples where it makes sense to optimize are checks for target thread to be current thread. In such cases there is no need to suspend the target thread, or use a VMop/HandshakeClosure. For instance, please, see the Monitor functions with the check: (java_thread == calling_thread). Getting information for current thread is frequently used case, e.g. to get info at an event point. 

Thanks,
Serguei


On 7/2/20 23:29, Yasumasa Suenaga wrote:

Hi Dan, David,

I uploaded new webrev. Could you review again?

http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.04/
OneGetThreadListStackTraces.java in this webrev would wait until thread state is transited to "waiting" with spin wait. 
CountDownLatch::await call as Dan pointed is fixed in it :)

Diff from webrev.03:
http://hg.openjdk.java.net/jdk/submit/rev/c9aeb7001e50


Thanks,

Yasumasa


On 2020/07/03 14:15, David Holmes wrote:



On 3/07/2020 2:27 pm, Yasumasa Suenaga wrote:

On 2020/07/03 12:24, Daniel D. Daugherty wrote:

On 7/2/20 10:50 PM, David Holmes wrote:
Sorry I'm responding here without seeing latest webrev but there is enough context I think ... 

On 3/07/2020 9:14 am, Yasumasa Suenaga wrote:

Hi Dan,

Thanks for your comment!

On 2020/07/03 7:16, Daniel D. Daugherty wrote:

On 7/2/20 5:19 AM, Yasumasa Suenaga wrote:

Hi David,

I upload new webrev. Could you review again?
http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.03/ 

src/hotspot/share/prims/jvmtiEnv.cpp
     L1542:     // Get stack trace with handshake
         nit - please add a period at the end.


I will fix it.



L1591: *stack_info_ptr = op.stack_info();
         The return parameter should not be touched unless the return 
         code in 'err' == JVMTI_ERROR_NONE.

     old L1582:   if (err == JVMTI_ERROR_NONE) {
         Please restore this check. The return parameter should not          be touched unless the return code in 'err' == JVMTI_ERROR_NONE. 

I will fix it.
But op.stack_info() will return NULL if the error is not JVMTI_ERROR_NONE. Are you (Dan) concerned about someone passing in a non-null/initialized out-pointer that will be reset to NULL if there was an error?
Actually the way we used to test this in POSIX tests is to call an API with known bad parameters and the return parameter ptr set to NULL. If the return parameter ptr was touched when an error should have been detected on an earlier parameter, then 
the test failed.
L1272:   if (!jt->is_exiting() && (thread_oop != NULL)) {          nit - extra parens around the second expression. 

I will fix it.



src/hotspot/share/prims/jvmtiEnvBase.cpp
     old L1532:   _result = JVMTI_ERROR_THREAD_NOT_ALIVE;          This deletion of the _result field threw me for a minute and then          I figured out that the field is init to JVMTI_ERROR_THREAD_NOT_ALIVE 
         in the constructor.
     L1553:   if (!jt->is_exiting() && (jt->threadObj() != NULL)) {          nit - extra parens around the second expression. 

I will fix it.



src/hotspot/share/prims/jvmtiEnvBase.hpp
     No comments.

src/hotspot/share/runtime/vmOperations.hpp
     No comments.
test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/GetThreadListStackTraces.java 
     No comments.
test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/OneGetThreadListStackTraces.java 
     L64: startSignal.countDown();
         I was expecting this to be a call to await() instead of 
         countDown(). What am I missing here?
         I think this test might be passing by accident right now, but...
Main thread (which call JVMTI functions to test) should wait until test thread is ready. So main thread would wait startSignal, and test thread would count down. 

No!
The test thread that previously called obj.wait() now calls latch.await().
The main thread that previously called obj.notify() now calls latch.countDown().
The main thread continues to spin until it sees the target is WAITING before proceeding with the test.
If I add spin wait to wait until transit target thread state is WAITING (as following), we don't need to call SuspendThread(). 
Which is better?
The original spin-wait loop checking for  WAITING is better because it is the only guarantee that the target thread is blocked where you need it to be. suspending the thread is racy as you don't know exactly where the suspend will hit. 

Thanks,
David
-----



```
/* Wait until the thread state transits to "waiting" */
while (th.getState() != Thread.State.WAITING) {
     Thread.onSpinWait();
}
```
For simplify, spin wait is prefer to OneGetThreadListStackTraces.java in webrev.03. 


Thanks,

Yasumasa



Here's the flow as I see it:

main thread
   - start worker thread
   - startSignal.await()
     - main is now blocked

worker thread
   - startSignal.countDown()
     - main is now unblocked
   - stopSignal.await()
     - worker is now blocked

main thread
   - checkCallStacks(th)
   - stopSignal.countDown()
     - worker is now unblocked
   - th.join
     - main is now blocked

worker thread
   - runs off the end of run()
     - main is now unblocked

main thread
   - run off the end of main()
test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libGetThreadListStackTraces.c      L92:   jthreads = (jthread *)malloc(sizeof(jthread) * num_threads); 
         You don't check for malloc() failure.
         'jthreads' is allocated but never freed.


I will fix it.
test/hotspot/jtreg/serviceability/jvmti/GetThreadListStackTraces/libOneGetThreadListStackTraces.c      L91:   result = (*jvmti)->SuspendThread(jvmti, thread);          Why are you suspending the thread? GetAllStackTraces() and GetThreadListStackTraces() do not require the target thread(s) 
         to be suspend.
         If you decide not to SuspendThread, then you don't need the 
         AddCapabilities or the ResumeThread calls.
Test thread might not be entered following code (stopSignal.await()). We might see deferent call stack between GetAllStackTraces() and GetThreadListStackTraces(). We cannot control to freeze call stack of test thread in Java code. (I didn't use SuspendThread() at first, but I saw some errors which causes in above.)
So we need to call SuspendThread() to ensure we can see same call stack.
If you are checking that the thread is in state WAITING then it cannot escape from that state and you can sample the stack multiple times from any API and get the same result.
I suspect the errors you saw were from the apparent incorrect use of the CountDownLatch.
With the flow outlined above, the worker thread should be nicely blocked in stopSignal.await() when stuff is sampled. 

Dan




Cheers,
David
-----



Thanks,

Yasumasa



Dan



On 2020/07/02 15:05, David Holmes wrote:

Hi Yasumasa,

On 1/07/2020 11:53 am, Yasumasa Suenaga wrote:

Hi,

I uploaded new webrev. Could review again?
http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.02/ 

Updates look fine - thanks.

One minor nit:

1274 _collector.allocate_and_fill_stacks(1);
1275 _collector.set_result(JVMTI_ERROR_NONE);
In the other places where you use _collector you rely on result being initialized to JVMTI_ERROR_NONE, rather than setting it directly after allocate_and_fill_stacks(). 

Fixed.



src/hotspot/share/prims/jvmtiEnvBase.cpp
  820 assert(SafepointSynchronize::is_at_safepoint() ||   821 java_thread->is_thread_fully_suspended(false, &debug_bits) ||   822 current_thread == java_thread->active_handshaker(),   823 "at safepoint / handshake or target thread is suspended");
I don't think the suspension check is necessary, as even if the target is suspended we must still be at a safepoint or in a handshake with it. Makes me wonder if we used to allow a racy stacktrace operation on a suspended thread, assuming it would remain suspended?
This function (JvmtiEnvBase::get_stack_trace()) can be called to get own stack trace. For example, we can call GetStackTrace() for current thread at JVMTI event. 
So I changed assert as below:

```
  820 assert(current_thread == java_thread ||
  821 SafepointSynchronize::is_at_safepoint() ||
  822 current_thread == java_thread->active_handshaker(),   823          "call by myself / at safepoint / at handshake"); 
```


Yep good catch. I hope current tests caught that.
They would be tested in vmTestbase/nsk/jvmti/GetStackTrace/getstacktr001/ (own call stacks), and getstacktr003 (call stacks in other thread). 



Speaking of tests ...
In the native code I think you need to check the success of all JNI methods that can throw exceptions - otherwise I believe the tests may trigger warnings if -Xcheck:jni is used with them. See for example:
test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/libHeapMonitorTest.cpp
I updated testcases to check JNI and JVMTI function calls. 



In the Java code the target thread:

   45     public void run() {
   46       try {
   47 synchronized (lock) {
   48 lock.wait();
   49 System.out.println("OK");
   50           }
is potentially susceptible to spurious wakeups. Using a CountDownLatch would be robust. 

Fixed.


Thanks,

Yasumasa



Thanks,
David
-----



Thanks,

Yasumasa


On 2020/07/01 8:48, David Holmes wrote:

Hi Yasumasa,

On 1/07/2020 9:05 am, Yasumasa Suenaga wrote:

Hi David,


1271 ResourceMark rm;
IIUC at this point the _calling_thread is the current thread, so we can use: 

ResourceMark rm(_calling_thread);
If so, we can call make_local() in L1272 without JavaThread (or we can pass current thread to make_local()). Is it right? 

```
1271 ResourceMark rm;
1272 _collector.fill_frames((jthread)JNIHandles::make_local(_calling_thread, thread_oop), 
1273 jt, thread_oop);
```
Sorry I got confused, _calling_thread may not be the current thread as we could be executing the handshake in the target thread itself. So the ResourceMark is correct as-is (implicitly for current thread).
The argument to fill_frames will be used in the jvmtiStackInfo and passed back to the _calling_thread, so it must be created via make_local(_calling_thread, ...) as you presently have. 

Thanks,
David


Thanks,

Yasumasa


On 2020/07/01 7:05, David Holmes wrote:

On 1/07/2020 12:17 am, Yasumasa Suenaga wrote:

Hi David,
Thank you for reviewing! I will update new webrev tomorrow.
466 class MultipleStackTracesCollector : public StackObj {
  498 class VM_GetAllStackTraces : public VM_Operation { 
  499 private:
  500 JavaThread *_calling_thread;
  501   jint _final_thread_count;
  502 MultipleStackTracesCollector _collector;
You can't have a StackObj as a member of another class like that as it may not be on the stack. I think MultipleStackTracesCollector should not extend any allocation class, and should always be embedded directly in another class. 

I'm not sure what does mean "embedded".
Is it ok as below?

```
class MultipleStackTracesCollector {
    :
}
class GetAllStackTraces : public VM_Operation { 
   private:
MultipleStackTracesCollector _collector;
}
```


Yes that I what I meant.

Thanks,
David
-----



Thanks,

Yasumasa


On 2020/06/30 22:22, David Holmes wrote:

Hi Yasumasa,
On 30/06/2020 10:05 am, Yasumasa Suenaga wrote: 
Hi David, Serguei,
I updated webrev for 8242428. Could you review again? This change migrate to use direct handshake for GetStackTrace() and GetThreadListStackTraces() (when thread_count == 1).
http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.01/
This looks really good now! I only have a few nits below. There is one thing I don't like about it but it requires a change to the main Handshake logic to address - in JvmtiEnv::GetThreadListStackTraces you have to create a ThreadsListHandle to convert the jthread to a JavaThread, but then the Handshake::execute_direct creates another ThreadsListHandle internally. That's a waste. I will discuss with Robbin and file a RFE to have an overload of execute_direct that takes an existing TLH. Actually it's worse than that because we have another TLH in use at the entry point for the JVMTI functions, so I think there may be some scope for simplifying the use of TLH instances - future RFE. 

---

src/hotspot/share/prims/jvmtiEnvBase.hpp
  451 GetStackTraceClosure(JvmtiEnv *env, jint start_depth, jint max_count,   452 jvmtiFrameInfo* frame_buffer, jint* count_ptr)   453     : HandshakeClosure("GetStackTrace"),   454 _env(env), _start_depth(start_depth), _max_count(max_count),   455 _frame_buffer(frame_buffer), _count_ptr(count_ptr), 
  456 _result(JVMTI_ERROR_THREAD_NOT_ALIVE) {
Nit: can you do one initializer per line please. 

This looks wrong:
466 class MultipleStackTracesCollector : public StackObj {
  498 class VM_GetAllStackTraces : public VM_Operation { 
  499 private:
  500 JavaThread *_calling_thread;
  501   jint _final_thread_count;
  502 MultipleStackTracesCollector _collector;
You can't have a StackObj as a member of another class like that as it may not be on the stack. I think MultipleStackTracesCollector should not extend any allocation class, and should always be embedded directly in another class.
481 MultipleStackTracesCollector(JvmtiEnv *env, jint max_frame_count) { 
  482     _env = env;
  483 _max_frame_count = max_frame_count;
  484 _frame_count_total = 0;
  485 _head = NULL;
  486 _stack_info = NULL;
  487 _result = JVMTI_ERROR_NONE;
  488   }
As you are touching this can you change it to use an initializer list as you did for the HandshakeClosure, and please keep one item per line. 

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src/hotspot/share/prims/jvmtiEnvBase.cpp
  820 assert(SafepointSynchronize::is_at_safepoint() ||   821 java_thread->is_thread_fully_suspended(false, &debug_bits) ||   822 current_thread == java_thread->active_handshaker(),   823 "at safepoint / handshake or target thread is suspended");
I don't think the suspension check is necessary, as even if the target is suspended we must still be at a safepoint or in a handshake with it. Makes me wonder if we used to allow a racy stacktrace operation on a suspended thread, assuming it would remain suspended? 

1268   oop thread_oop = jt->threadObj();
1269
1270   if (!jt->is_exiting() && (jt->threadObj() != NULL)) { 

You can use thread_oop in line 1270.
1272 _collector.fill_frames((jthread)JNIHandles::make_local(_calling_thread, thread_oop), 
1273 jt, thread_oop);
It is frustrating that this entire call chain started with a jthread reference, which we converted to a JavaThread, only to eventually need to convert it back to a jthread! I think there is some scope for simplification here but not as part of this change. 

1271 ResourceMark rm;
IIUC at this point the _calling_thread is the current thread, so we can use: 

ResourceMark rm(_calling_thread);

---

Please add @bug lines to the tests.
I'm still pondering the test logic but wanted to send this now. 

Thanks,
David
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VM_GetThreadListStackTrace (for GetThreadListStackTraces) and VM_GetAllStackTraces (for GetAllStackTraces) have inherited VM_GetMultipleStackTraces VM operation which provides the feature to generate jvmtiStackInfo. I modified VM_GetMultipleStackTraces to a normal C++ class to share with HandshakeClosure for GetThreadListStackTraces (GetSingleStackTraceClosure).
Also I added new testcases which test GetThreadListStackTraces() with thread_count == 1 and with all threads.
This change has been tested in serviceability/jvmti serviceability/jdwp vmTestbase/nsk/jvmti vmTestbase/nsk/jdi vmTestbase/nsk/jdwp. 


Thanks,

Yasumasa


On 2020/06/24 15:50, Yasumasa Suenaga wrote:

Hi all,

Please review this change:
   JBS: https://bugs.openjdk.java.net/browse/JDK-8242428    webrev: http://cr.openjdk.java.net/~ysuenaga/JDK-8242428/webrev.00/
This change replace following VM operations to direct handshake. 

  - VM_GetFrameCount (GetFrameCount())
  - VM_GetFrameLocation (GetFrameLocation())   - VM_GetThreadListStackTraces (GetThreadListStackTrace()) 
  - VM_GetCurrentLocation
GetThreadListStackTrace() uses direct handshake if thread count == 1. In other case (thread count > 1), it would be performed as VM operation (VM_GetThreadListStackTraces). Caller of VM_GetCurrentLocation (JvmtiEnvThreadState::reset_current_location()) might be called at safepoint. So I added safepoint check in its caller.
This change has been tested in serviceability/jvmti serviceability/jdwp vmTestbase/nsk/jvmti vmTestbase/nsk/jdi vmTestbase/ns 
k/jdwp.
Also I tested it on submit repo, then it has execution error (mach5-one-ysuenaga-JDK-8242428-20200624-0054-12034717) due to dependency error. So I think it does not occur by this change. 


Thanks,

Yasumasa

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