Integrated: 8257831: Suspend with handshakes

[Robbin Ehn]

On Thu, 25 Mar 2021 10:56:23 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...

This pull request has now been integrated.

Changeset: 86bd44fe
Author:    Robbin Ehn <rehn at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/86bd44fe
Stats:     1349 lines in 40 files changed: 271 ins; 882 del; 196 mod

8257831: Suspend with handshakes

Reviewed-by: dcubed, rrich, dholmes, pchilanomate, sspitsyn

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

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