RFR 8230594: Allow direct handshakes without VMThread intervention

David Holmes david.holmes at oracle.com
Thu Jan 16 04:18:20 UTC 2020


v3 looks good!

Time for Robbin and Dan to jump in :)

Thanks,
David

On 16/01/2020 1:56 pm, Patricio Chilano wrote:
> Hi David,
> 
> On 1/15/20 8:29 PM, David Holmes wrote:
>> Hi Patricio,
>>
>> On 16/01/2020 6:06 am, Patricio Chilano wrote:
>>> Hi David,
>>>
>>> Here is v2:
>>>
>>> Full: http://cr.openjdk.java.net/~pchilanomate/8230594/v02/webrev/
>>> Inc: http://cr.openjdk.java.net/~pchilanomate/8230594/v02/inc/webrev/
>>
>> src/hotspot/share/runtime/handshake.cpp
>>
>> why is it that execute_direct doesn't check for 
>> uses_thread_local_poll() ?
> Sorry, I thought for a moment that thread-local handshakes for arm32 was 
> already implemented and all platforms where using it. I added that check 
> back, and by testing that path I had to also add back the VMThread in 
> the assert in walk_stack_and_revoke().
> 
>> src/hotspot/share/runtime/handshake.hpp
>>
>> typo: wether -> whether
> Fixed.
> 
> Here is v3:
> Full: http://cr.openjdk.java.net/~pchilanomate/8230594/v03/webrev/
> Inc: http://cr.openjdk.java.net/~pchilanomate/8230594/v03/inc/webrev/
> 
> Thanks!
> 
> Patricio
>> Otherwise updates look fine.
>>
>> Thanks,
>> David
>>
>>> Passed one round of t1-6, and running more rounds now.
>>>
>>> Thanks,
>>> Patricio
>>>
>>> On 1/14/20 7:57 PM, David Holmes wrote:
>>>> Hi Patricio,
>>>>
>>>> On 15/01/2020 5:17 am, Patricio Chilano wrote:
>>>>> Hi David,
>>>>>
>>>>> On 1/14/20 4:13 AM, David Holmes wrote:
>>>>>> Hi Patricio,
>>>>>>
>>>>>> Thanks for tackling this. I've taken an initial look through 
>>>>>> trying to understand the general approach, but I don't claim to 
>>>>>> understand all the subtle details, so there are some queries below 
>>>>>> to help with my overall understanding.
>>>>>>
>>>>>> On 14/01/2020 2:25 am, Patricio Chilano wrote:
>>>>>>> Hi all,
>>>>>>>
>>>>>>> The following patch adds the ability to execute direct handshakes 
>>>>>>> between JavaThreads without the VMThread intervention, and 
>>>>>>> enables this functionality for biased locking revocations.
>>>>>>> The current handshake mechanism that uses the VMThread, either to 
>>>>>>> handshake one JavaThread or all of them, is still the default 
>>>>>>> unless you specify otherwise when calling Handshake::execute().
>>>>>>
>>>>>> Can I suggest that rather than using an overloaded:
>>>>>>
>>>>>>  static bool execute(HandshakeClosure* hs_cl, JavaThread* target, 
>>>>>> bool is_direct_handshake = false);
>>>>>>
>>>>>> you instead add an execute_direct method for clarity?
>>>>> Ok, added execute_direct() and removed overloading.
>>>>>
>>>>>>> In order to avoid adding additional overhead to this path that 
>>>>>>> uses the VMThread (especially the one that handshakes all 
>>>>>>> JavaThreads) I added a new HandshakeOperation pointer in the 
>>>>>>> HandshakeState class, _operation_direct, to be used for the 
>>>>>>> direct handshake cases only and whose access is serialized 
>>>>>>> between JavaThreads by using a semaphore. Thus, one direct 
>>>>>>> handshake will be allowed at any given time, and upon completion 
>>>>>>> the semaphore will be signaled to allow the next handshaker if 
>>>>>>> any to proceed. In this way the old _operation can still be used 
>>>>>>> only by the VMThread without the need for synchronization to 
>>>>>>> access it. The handshakee will now check if any of _operation or 
>>>>>>> _operation_direct is set when checking for a pending handshake 
>>>>>>> and will try to execute both in 
>>>>>>> HandshakeState::process_self_inner(). The execution of the 
>>>>>>> handshake’s ThreadClosure, either direct handshake or not, is 
>>>>>>> still protected by a semaphore, which I renamed to _processing_sem.
>>>>>>> I converted the semaphore _done in HandshakeOperation to be just 
>>>>>>> an atomic counter because of bug 
>>>>>>> https://sourceware.org/bugzilla/show_bug.cgi?id=12674 (which I 
>>>>>>> actually hit once!). Since the semaphore could not be static 
>>>>>>> anymore due to possibly having more than one HandshakeOperation 
>>>>>>> at a time, the handshakee could try to access the nwaiters field 
>>>>>>> of an already destroyed semaphore when signaling it. In any case 
>>>>>>> nobody was waiting on that semaphore (we were not using kernel 
>>>>>>> functionality), so just using an atomic counter seems more 
>>>>>>> appropriate.
>>>>>>> In order to avoid issues due to disarming a JavaThread that 
>>>>>>> should still be armed for a handshake or safepoint, each 
>>>>>>> JavaThread will now always disarm its own polling page.
>>>>>>> I also added a new test, HandshakeDirectTest.java, which tries to 
>>>>>>> stress the use of direct handshakes with revocations.
>>>>>>> In terms of performance, I measured no difference in the 
>>>>>>> execution time of one individual handshake. The difference can be 
>>>>>>> seen when several handshakes at a time are executed as expected. 
>>>>>>> So for example on Linux running on an Intel Xeon 8167M cpu, test 
>>>>>>> HandshakeDirectTest.java (which executes 50000 handshakes between 
>>>>>>> 32 threads) executes in around 340ms using direct handshakes and 
>>>>>>> in around 5.6 seconds without it. For a modified version of that 
>>>>>>> test that only executes 128 handshakes between the 32 threads and 
>>>>>>> avoids any suspend-resume, the test takes around 12ms with direct 
>>>>>>> handshakes and 19ms without it.
>>>>>>> Tested with mach5, tiers1-6.
>>>>>>>
>>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8230594
>>>>>>> Webrev: http://cr.openjdk.java.net/~pchilanomate/8230594/v01/webrev/
>>>>>>
>>>>>> src/hotspot/share/runtime/handshake.hpp
>>>>>>
>>>>>>  36 // A handshake closure is a callback that is executed for each 
>>>>>> JavaThread
>>>>>>   37 // while that thread is in a safepoint safe state. The 
>>>>>> callback is executed
>>>>>>   38 // either by the thread itself or by the VM thread while 
>>>>>> keeping the thread
>>>>>>   39 // in a blocked state. A handshake can be performed with a 
>>>>>> single
>>>>>>   40 // JavaThread as well.
>>>>>>
>>>>>> Does this comment block need updating for the direct case?
>>>>>>
>>>>>>  61 // the operation is only done by either VM thread on behalf of 
>>>>>> the JavaThread
>>>>>>  62 // or the JavaThread itself.
>>>>>>
>>>>>> Again does this need an update?
>>>>> Yes, I forgot to update those comments in handshake.hpp. Fixed.
>>>>>
>>>>>
>>>>>> src/hotspot/share/runtime/handshake.hpp
>>>>>>
>>>>>>  41 class HandshakeOperation: public StackObj {
>>>>>>
>>>>>> Would it be clearer to have HandShakeOperation subclassed by 
>>>>>> DirectHandShakeOperation, rather than it being a property of the 
>>>>>> current op instance?
>>>>> But I would still need to access the is_direct field for non-direct 
>>>>> handshakes in HandshakeState::try_process() anyways.
>>>>
>>>> Okay.
>>>>
>>>>>
>>>>>>  41 class HandshakeOperation: public StackObj {
>>>>>>  42   HandshakeClosure* _handshake_cl;
>>>>>>  43   int64_t _pending_threads;
>>>>>>  ...
>>>>>>  53   void add_target_count(int count) { 
>>>>>> Atomic::add(&_pending_threads, count); }
>>>>>>
>>>>>> Can you clarify the lifecycle of this StackObj please. You 
>>>>>> obviously expose it to other threads - hence the atomic update - 
>>>>>> but that means it needs to be guaranteed to be live when that 
>>>>>> update occurs.
>>>>> The HandshakeOperation object is created in Handshake::execute() 
>>>>> (or Handshake::execute_direct() now) by the Handshaker. The only 
>>>>> other threads that could access this object are the VMThread(for 
>>>>> non-direct case only) and the Handshakee(in both cases). Seeing 
>>>>> that it's safe for the VMThread to decrement the counter is 
>>>>> straightforward since the Handshaker will be waiting on the 
>>>>> VMOperationRequest_lock waiting for the operation to finish (this 
>>>>> is just as always has been).
>>>>> As for the Handshakee, we can see that the HandshakeOperation 
>>>>> object will be alive as long as the VMThread(for the non-direct 
>>>>> case) or Handshaker(for the direct case) keep seeing that the 
>>>>> operation is not done by calling 
>>>>> HandshakeOperation::is_completed(). Once that returns true, for the 
>>>>> non-direct case the VMThread will finish the operation and wake up 
>>>>> the Handshaker who will return from Handshake::execute() and the 
>>>>> object will be destroyed, and for the direct case the Handshaker 
>>>>> will just return from execute_direct() and the object will be 
>>>>> destroyed. Since is_completed() will only return true when 
>>>>> _pending_threads reaches zero, that means the decrement operation 
>>>>> had to be safe. Just as an observation, for the HandshakeAllThreads 
>>>>> operation, _pending_threads could go to zero and even negative 
>>>>> before executing add_target_count(), but that's okay because the 
>>>>> VMThread doesn't call is_completed() before that and the load of 
>>>>> _pending_threads cannot float above add_target_count() since the 
>>>>> atomic add operation provides a memory fence.
>>>>> And finally the Handshakee cannot try to execute an operation that 
>>>>> has already being processed by the VMThread/Handshaker, because 
>>>>> they clear it first and then signal the _processing_sem semaphore 
>>>>> in HandshakeState::try_process(). That worked the same before this 
>>>>> change.
>>>>
>>>> Thanks for the detailed explanation!
>>>>
>>>>>
>>>>>> -- 
>>>>>>
>>>>>>  349     HandshakeOperation* op = Atomic::load_acquire(&_operation);
>>>>>>  350     if (op == NULL) {
>>>>>>  351       op = Atomic::load_acquire(&_operation_direct);
>>>>>>
>>>>>> This gives preference to the non-direct op. Is it possible for the 
>>>>>> direct-op to be indefinitely delayed if there is a series of 
>>>>>> non-direct ops?
>>>>> Yes, it's possible, although I think that's an unlikely scenario. 
>>>>> That would mean the Handshakee would be stuck in that while loop 
>>>>> with the VMThread constantly setting new operations in _operation. 
>>>>> But I can modify the loop and make it try to execute both before 
>>>>> going into the next interation, something like (more on 
>>>>> load_acquire next):
>>>>>
>>>>>      if (has_operation()) {
>>>>>        HandleMark hm(_thread);
>>>>>        CautiouslyPreserveExceptionMark pem(_thread);
>>>>>        HandshakeOperation * op = _operation;
>>>>>        if (op != NULL) {
>>>>>          // Disarm before execute the operation
>>>>>          clear_handshake(false);
>>>>>          op->do_handshake(_thread);
>>>>>        }
>>>>>        op = _operation_direct;
>>>>>        if (op != NULL) {
>>>>>          // Disarm before execute the operation
>>>>>          clear_handshake(true);
>>>>>          op->do_handshake(_thread);
>>>>>        }
>>>>>      }
>>>>>
>>>>> what do you think?
>>>>
>>>> That seems better to me.
>>>>
>>>>>
>>>>>> Also I don't see the release_stores that these load_acquire would 
>>>>>> pair with. I'm assuming it should be here:
>>>>>>
>>>>>>  319     _operation = op;
>>>>>>  323     _operation_direct = op;
>>>>>>
>>>>>> But I would also have expected all necessary memory 
>>>>>> synchronization to already be present via the semaphore operations 
>>>>>> and/or the rest of the handshake mechanism. ??
>>>>> Yes, the release is in SafepointMechanism::arm_local_poll_release() 
>>>>> after setting those fields. But it's true that those load_acquire 
>>>>> are not needed since the semaphore already has acquire semantics. I 
>>>>> actually wanted to do a normal load but ended up just copying how 
>>>>> we were loading _operation. I''ll change them and retest.
>>>>
>>>> Okay.
>>>>
>>>> Thanks,
>>>> David
>>>> -------
>>>>
>>>>>
>>>>>> 396 if ((!is_direct && _operation != NULL) || (is_direct && 
>>>>>> _operation_direct != NULL)){
>>>>>> 406   if ((!is_direct && _operation == NULL) || (is_direct && 
>>>>>> _operation_direct == NULL)){
>>>>>>
>>>>>> Can this not be abstracted back into a "has_operation" method that 
>>>>>> takes a "direct" parameter? e.g
>>>>>>
>>>>>>   bool has_operation() const { return _operation != NULL || 
>>>>>> _operation_direct != NULL; }
>>>>>> + bool has_specific_operation(bool direct) {
>>>>>> +   return direct ? _operation_direct != NULL : _operation != NULL;
>>>>>> + }
>>>>> Done.
>>>>>
>>>>>
>>>>>> ---
>>>>>>
>>>>>> test/hotspot/jtreg/runtime/handshake/HandshakeDirectTest.java
>>>>>>
>>>>>> Please add:
>>>>>>
>>>>>> import java.util.concurrent.Semaphore;
>>>>>>
>>>>>> so you can just refer to Sempahore.
>>>>> Done.
>>>>>
>>>>>
>>>>>> Style nit: _working_threads
>>>>>>
>>>>>> Java style is to not use leading underscore and to use camelCase 
>>>>>> for variables ie workingThreads.
>>>>> Right, I changed them to camelCase.
>>>>>
>>>>>
>>>>>> 44 static Thread _suspendresume_thread = new Thread();
>>>>>>
>>>>>> The above is dead code.
>>>>> Removed.
>>>>>
>>>>>
>>>>>> 53 if (_is_biased[me] == false) {
>>>>>>
>>>>>> Style nit: use "if (!_is_biased[me]) {"
>>>>> Changed.
>>>>>
>>>>>
>>>>>> 80             } catch(InterruptedException ie) {
>>>>>>   81             }
>>>>>>
>>>>>> I suggest inserting "throw new Error("Unexpected interrupt");" for 
>>>>>> good measure.
>>>>>>
>>>>>>  111                     _working_threads[i].suspend();
>>>>> Done.
>>>>>
>>>>>
>>>>>> Thread suspension has been deprecated-for-removal as of JDK 14 so 
>>>>>> could be gone in JDK 15 (but more likely 16). If the 
>>>>>> suspend/resume is important for this test then you will need to 
>>>>>> switch to using JVM TI suspend/resume; or else perhaps introduce a 
>>>>>> WhiteBox method to do whatever you need in the VM.
>>>>> It's not really needed for this test. I just wanted to mixed 
>>>>> suspend-resume with handshakes because I know there could be some 
>>>>> buggy interactions between them. But since it has been deprecated 
>>>>> then maybe there is no point in stressing handshakes with them and 
>>>>> I can remove that part.
>>>>>
>>>>>
>>>>>>  123 // Wait until the desired number of direct handshakes is reached
>>>>>>  124         while (_handshake_count.get() < 
>>>>>> DIRECT_HANDSHAKES_MARK) {
>>>>>>  125             Thread.sleep(10); // sleep for 10ms
>>>>>>  126         }
>>>>>>
>>>>>> You could just do a join() on one of the worker threads.
>>>>> Right, didn't thought about that. Changed.
>>>>>
>>>>>
>>>>> Thanks for looking at this David! I'll retest with the above 
>>>>> changes in the meantime.
>>>>>
>>>>>
>>>>> Patricio
>>>>>> Thanks,
>>>>>> David
>>>>>> -----
>>>>>>
>>>>>>> Thanks!
>>>>>>> Patricio
>>>>>>>
>>>>>
>>>
> 


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