On Wed, 31 Mar 2021 08:27:35 GMT, Robbin Ehn <r...@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) Hi Robbin, Great work! I only have minor comments. ------------- Marked as reviewed by pchilanomate (Committer). PR: https://git.openjdk.java.net/jdk/pull/3191