RFR: 8257831: Suspend with handshakes [v3]

[Patricio Chilano Mateo]

On Wed, 31 Mar 2021 08:27:35 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> A suspend request is done by handshaking thread target thread(s). When executing the handshake operation we know the target mutator thread is in a dormant state (as in safepoint safe state). We have a guarantee that it will check it's poll before leaving the dormant state. To stop the thread from leaving the the dormant state we install a second asynchronous handshake to be executed by the targeted thread. The asynchronous handshake will wait on a monitor while the thread is suspended. The target thread cannot not leave the dormant state without a resume request.
>> 
>> Per thread suspend requests are naturally serialized by the per thread HandshakeState lock (we can only execute one handshake at a time per thread).
>> Instead of having a separate lock we use this to our advantage and use HandshakeState lock for serializing access to the suspend flag and for wait/notify. 
>> 
>> Suspend:
>> Requesting thread -> synchronous handshake -> target thread
>> Inside synchronus handshake (HandshakeState lock is locked while
>> executing any handshake):
>> 	- Set suspended flag
>> 	- Install asynchronous handshake
>> 
>> Target thread -> tries to leave dormant state -> Executes handshakes
>> Target only executes asynchronous handshake:
>> 	- While suspended
>> 	- Go to blocked
>> 	- Wait on HandshakeState lock
>> 
>> Resume:
>> Resuming thread:
>> 	- Lock HandshakeState lock
>> 	- Clear suspended flag
>> 	- Notify HandshakeState lock
>> 	- Unlock HandshakeState lock
>> 
>> The "suspend requested" flag is an optimization, without it a dormant thread could be suspended and resumed many times and each would add a new asynchronous handshake. Suspend requested flag means there is already an asynchronous suspend handshake in queue which can be re-used, only the suspend flag needs to be set.
>> 
>> ----
>> Some code can be simplified or done in a smarter way but I refrained from doing such changes instead tried to keep existing code as is as far as possible. This concerns especially raw monitors.
>> 
>> ----
>> Regarding the changed test, the documentation says:
>> "If the calling thread is specified in the request_list array, this function will not return until some other thread resumes it."
>> 
>> But the code:
>>   LOG("suspendTestedThreads: before JVMTI SuspendThreadList");
>>   err = jvmti->SuspendThreadList(threads_count, threads, results);
>>   ...
>>   // Allow the Main thread to inspect the result of tested threads suspension	
>>   agent_unlock(jni);
>>   
>> The thread will never return from SuspendThreadList until resumed, so it cannot unlock with agent_unlock().
>> Thus main thread is stuck forever on:
>>   // Block until the suspender thread competes the tested threads suspension	
>>   agent_lock(jni);
>> 
>> And never checks and resumes the threads. So I removed that lock instead just sleep and check until all thread have the expected suspended state.
>> 
>> ----
>> 
>> This version already contains updates after pre-review comments from @dcubed-ojdk, @pchilano, @coleenp.
>> (Pre-review comments here:
>> https://github.com/openjdk/jdk/pull/2625)
>> 
>> ---- 
>> Testing t1-t8, nsk_jdi/nsk_jvmti/jdk_jdi/tck, KS24, RunThese and
>> combinations like running with -XX:ZCollectionInterval=0.01 -
>> XX:ZFragmentationLimit=0.
>> Running above some of above concurrently (load ~240), slow debug,
>> etc...
>
> Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - Merge branch 'master' into SuspendInHandshake
>  - Merge branch 'master' into SuspendInHandshake
>  - 8257831: Suspend with handshake (review baseline)

src/hotspot/share/runtime/handshake.hpp line 151:

> 149: 
> 150:   bool is_suspended()                 { return Atomic::load(&_suspended); }
> 151:   void set_suspend(bool to)           { return Atomic::store(&_suspended, to); }

Maybe set_suspended() instead()?

src/hotspot/share/runtime/objectMonitor.cpp line 436:

> 434:     current->set_current_pending_monitor(NULL);
> 435: 
> 436:     // We cleared the pending monitor info since we've just gotten past

Comment below needs updating since it mentions ThreadBlockInVM but we are not using it anymore.

src/hotspot/share/runtime/thread.cpp line 277:

> 275: #endif
> 276: 
> 277:   _SR_lock = new Monitor(Mutex::suspend_resume, "SR_lock", true,

Mutex::suspend_resume rank is not used by any other monitor so we could remove it.

src/hotspot/share/runtime/thread.hpp line 1142:

> 1140:   bool java_resume();  // higher-level resume logic called by the public APIs
> 1141:   bool is_suspended()                 { return _handshake.is_suspended(); }
> 1142:   bool suspend_request_pending()      { return _handshake.suspend_request_pending(); }

If you use is_suspended() to check for suspension in objectMonitor::enter() then we can remove this method.

-------------

PR: https://git.openjdk.java.net/jdk/pull/3191