RFR: 8257831: Suspend with handshakes [v9]

David Holmes dholmes at openjdk.java.net
Thu Apr 15 22:27:01 UTC 2021


On Thu, 15 Apr 2021 07:14:09 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 13 commits:
> 
>  - Merge branch 'master' into SuspendInHandshake
>  - Review fixes 4
>  - Fixed flag undef dep + spelling error
>  - Obsolete unused flags
>  - Review fixes 3
>  - Merge branch 'master' into SuspendInHandshake
>  - Review fixes 2
>  - White space fixes
>  - Merge branch 'master' into SuspendInHandshake
>  - Review fixes
>  - ... and 3 more: https://git.openjdk.java.net/jdk/compare/b224b566...27bf041c

Only nits on the incremental.

Like Dan I need to re-examine the complete set of changes to see if anything else sticks out (other than the non-encapsulation of the state changes around locking/unlocking when suspended).

Thanks,
David

src/hotspot/share/prims/jvmtiEnv.cpp line 952:

> 950:   if (!JvmtiSuspendControl::suspend(java_thread)) {
> 951:     // Either the thread is already suspended or
> 952:     // it was in process of exiting.

Nit: the previous comment was perfectly correct. It is okay to replace the second "the thread" with "it" but it should still read "in _the_ process of exiting".

src/hotspot/share/prims/jvmtiEnv.cpp line 993:

> 991:     if (!JvmtiSuspendControl::suspend(java_thread)) {
> 992:       // Either the thread is already suspended or
> 993:       // it was in process of exiting.

Ditto - "in the process ..."

src/hotspot/share/prims/jvmtiEnv.cpp line 1006:

> 1004:     if (!JvmtiSuspendControl::suspend(current)) {
> 1005:       // Either the thread is already suspended or
> 1006:       // it was in process of exiting.

Ditto - "in the process ..."

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

Marked as reviewed by dholmes (Reviewer).

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


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