RFR: 8257831: Suspend with handshakes [v6]

[Daniel D.Daugherty]

On Mon, 12 Apr 2021 10:53:46 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 eight commits:
> 
>  - Merge branch 'master' into SuspendInHandshake
>  - Review fixes 2
>  - White space fixes
>  - Merge branch 'master' into SuspendInHandshake
>  - Review fixes
>  - Merge branch 'master' into SuspendInHandshake
>  - Merge branch 'master' into SuspendInHandshake
>  - 8257831: Suspend with handshake (review baseline)

I only found nits so still looks good to me.

src/hotspot/share/runtime/handshake.cpp line 466:

> 464:     // The exception to this rule is the asynchronous suspension handshake.
> 465:     // It by-passes the NSV by manully doing the transition.
> 466:     // Since we can have safepoints while suspeneded we need to release the tty locker if it is held.

nit typo: s/manully/manually/
nit typo: s/suspeneded/suspended/

src/hotspot/share/runtime/handshake.hpp line 133:

> 131:     // check the _lock, if held go to slow path.
> 132:     // Since the handshakee is unsafe if _lock gets lock after this check
> 133:     // we know another threads cannot process any handshakes.

nit typo: s/_lock gets lock/_lock gets locked/
nit typo: s/threads/thread/

src/hotspot/share/runtime/thread.cpp line 1448:

> 1446:     // The careful dance between thread suspension and exit is handled here.
> 1447:     // Since we are in vm and suspension is done with handshakes,
> 1448:     // we can just put in the exiting state and it will be corrcetly handled.

Perhaps: s/Since we are in vm and/Since we are in thread_in_vm state and/
nit typo: s/corrcetly/correctly/

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

Marked as reviewed by dcubed (Reviewer).

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