The following test timed out in the JDK16 CI:

vmTestbase/nsk/jdi/HiddenClass/events/events001.java

Here's a snippet from the log file:

#section:main
----------messages:(7/1158)----------
command: main nsk.jdi.HiddenClass.events.events001 -verbose -arch=linux-x64 -waittime=5 -debugee.vmkind=java -transport.address=dynamic -debugee.vmkeys="-Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI -XX:MaxRAMPercentage=6 -Djava.io.tmpdir=/opt/mach5/mesos/work_dir/slaves/983c483a-6907-44e0-ad29-98c7183575e2-S14479/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/61274ca5-cc43-408f-bd31-6a616fecf2f9/runs/d119d9cd-fa27-44a5-99a5-e715eea2c28d/testoutput/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jdi/tmp -server"
reason: User specified action: run main/othervm nsk.jdi.HiddenClass.events.events001 -verbose -arch=${os.family}-${os.simpleArch} -waittime=5 -debugee.vmkind=java -transport.address=dynamic -debugee.vmkeys="-Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI ${test.vm.opts} ${test.java.opts}"
Mode: othervm [/othervm specified]
Additional options from @modules: --add-modules java.base --add-exports java.base/jdk.internal.misc=ALL-UNNAMED --add-opens java.base/jdk.internal.misc=ALL-UNNAMED
Timeout information:
--- Timeout information end.
elapsed time (seconds): 510.183
----------configuration:(5/166)----------
Boot Layer
  add modules: java.base
  add exports: java.base/jdk.internal.misc ALL-UNNAMED
  add opens: java.base/jdk.internal.misc ALL-UNNAMED

----------System.out:(111/7189)*----------

# Connecting to debuggee
binder> VirtualMachineManager: version 16.0
binder> Finding connector: default
binder> LaunchingConnector:
binder> name: com.sun.jdi.CommandLineLaunch
binder> description: Launches target using Sun Java VM command line and attaches to it
binder> transport: com.sun.tools.jdi.SunCommandLineLauncher$2@73e376ff
binder> Connector arguments:
binder> home=/opt/mach5/mesos/work_dir/jib-master/install/jdk-16+27-1884/linux-x64-debug.jdk/jdk-16/fastdebug
binder> vmexec=java
binder> options=-Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI -XX:MaxRAMPercentage=6 -Djava.io.tmpdir=/opt/mach5/mesos/work_dir/slaves/983c483a-6907-44e0-ad29-98c7183575e2-S14479/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/61274ca5-cc43-408f-bd31-6a616fecf2f9/runs/d119d9cd-fa27-44a5-99a5-e715eea2c28d/testoutput/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jdi/tmp -server
binder> main=nsk.jdi.HiddenClass.events.events001a \u0000-verbose\u0000 \u0000-arch=linux-x64\u0000 \u0000-waittime=5\u0000 \u0000-debugee.vmkind=java\u0000 \u0000-transport.address=dynamic\u0000 \u0000-debugee.vmkeys="-Xbootclasspath/a:. -XX:+UnlockDiagnosticVMOptions -XX:+WhiteBoxAPI -XX:MaxRAMPercentage=6 -Djava.io.tmpdir=/opt/mach5/mesos/work_dir/slaves/983c483a-6907-44e0-ad29-98c7183575e2-S14479/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/61274ca5-cc43-408f-bd31-6a616fecf2f9/runs/d119d9cd-fa27-44a5-99a5-e715eea2c28d/testoutput/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jdi/tmp -server"\u0000 \u0000-pipe.port=55387\u0000
binder> quote=\u0000
binder> suspend=true
binder> Launching debugee
binder> Waiting for VM initialized
Initial VMStartEvent received: VMStartEvent in thread main
# Resuming debuggee

# EventHandler is started

# Waiting for command: ready

EventHandler started
debugee.stderr>
debugee.stderr> Debuggee: started
debugee.stderr> Debuggee: Sending command: ready
debugee.stderr> Debuggee: Waiting for command: run

# Got command: ready

# Creating ClassPrepareRequest
  Adding filter to ClassPrepareRequest: nsk.jdi.HiddenClass.events.HiddenClass/0x*
  Enabled ClassPrepareRequest

# Creating request for ClassUnloadEvent
  Adding filter to ClassUnloadRequest: nsk.jdi.HiddenClass.events.HiddenClass/0x*
  Enabled ClassUnloadRequest

# Sending command: run

# Waiting for command: ready
debugee.stdout> Try: /opt/mach5/mesos/work_dir/slaves/983c483a-6907-44e0-ad29-98c7183575e2-S14479/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/61274ca5-cc43-408f-bd31-6a616fecf2f9/runs/d119d9cd-fa27-44a5-99a5-e715eea2c28d/testoutput/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jdi/classes/3/vmTestbase/nsk/jdi/HiddenClass/events/events001.d/nsk/jdi/HiddenClass/events/HiddenClass.class
debugee.stdout> Try: /opt/mach5/mesos/work_dir/jib-master/install/jdk-16+27-1884/src.full/open/test/hotspot/jtreg/vmTestbase/nsk/jdi/HiddenClass/events/nsk/jdi/HiddenClass/events/HiddenClass.class
debugee.stdout> Try: /opt/mach5/mesos/work_dir/slaves/983c483a-6907-44e0-ad29-98c7183575e2-S14479/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/61274ca5-cc43-408f-bd31-6a616fecf2f9/runs/d119d9cd-fa27-44a5-99a5-e715eea2c28d/testoutput/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jdi/classes/3/vmTestbase/nsk/jdi/HiddenClass/events/HiddenClass.class

ClassPrepareEvent: ClassPrepareEvent in thread main
ClassPrepareEvent class name: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00
ClassPrepareEvent class sign: Lnsk/jdi/HiddenClass/events/HiddenClass.0x0000000801014c00;
debugee.stderr> Debuggee: defined a hidden class: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00
debugee.stderr> Debuggee: created an instance of a hidden class: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00
debugee.stderr> Debuggee: Sending command: ready

# Got command: ready
debugee.stderr> Debuggee: Waiting for command: run
  Found method: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00::hcMethod

# Creating BreakpointRequest
  Enabled BreakpointRequest
  Found field: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00::hcField

# Creating request for ModificationWatchpointRequest
  Adding filter to ModificationWatchpointRequest: nsk.jdi.HiddenClass.events.HiddenClass/0x*
  Enabled ModificationWatchpointRequest

# Sending command: run

# Waiting for command: done
debugee.stderr> Debuggee: invoking a method of a hidden class: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00

BreakpointEvent: BreakpointEvent@nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00:50 in thread main
BreakpointEvent: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00::hcMethod
  Found method: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00::getHCField
  invoking method: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00.getHCField()
   method getHCField returned result: "<Not initialized>"

ModificationWatchpointEvent: ModificationWatchpoint@nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00:50 in thread main
ModificationWatchpointEvent: field: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00::hcField
ModificationWatchpointEvent: value: "nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801014c00"
debugee.stderr> Debuggee: started provoking class unload events
debugee.stdout> Try: /opt/mach5/mesos/work_dir/slaves/983c483a-6907-44e0-ad29-98c7183575e2-S14479/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/61274ca5-cc43-408f-bd31-6a616fecf2f9/runs/d119d9cd-fa27-44a5-99a5-e715eea2c28d/testoutput/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jdi/classes/3/vmTestbase/nsk/jdi/HiddenClass/events/events001.d/nsk/jdi/HiddenClass/events/HiddenClass.class
debugee.stdout> Try: /opt/mach5/mesos/work_dir/jib-master/install/jdk-16+27-1884/src.full/open/test/hotspot/jtreg/vmTestbase/nsk/jdi/HiddenClass/events/nsk/jdi/HiddenClass/events/HiddenClass.class
debugee.stdout> Try: /opt/mach5/mesos/work_dir/slaves/983c483a-6907-44e0-ad29-98c7183575e2-S14479/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/61274ca5-cc43-408f-bd31-6a616fecf2f9/runs/d119d9cd-fa27-44a5-99a5-e715eea2c28d/testoutput/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_nsk_jdi/classes/3/vmTestbase/nsk/jdi/HiddenClass/events/HiddenClass.class
debugee.stderr> Debuggee: finished provoking class unload events
debugee.stderr> Debuggee: Sending command: done

# Got command: done

# Waiting for EventHandler to complete
debugee.stderr> Debuggee: Waiting for command: quit
Timeout refired 480 times

ClassUnloadEvent class name: nsk.jdi.HiddenClass.events.HiddenClass/0x0000000801015000
ClassUnloadEvent class sign: Lnsk/jdi/HiddenClass/events/HiddenClass.0x0000000801015000;

EventHandler finished
# EventHandler completed
  Disabling BreakpointRequest
  Disabling ClassPrepareRequest
  Disabling ClassUnloadRequest
  Disabling ModificationWatchpointRequest

# Sending command: quit

# Shutting down debuggee
# Waiting for debuggee terminating
debugee.stderr>
debugee.stderr> Debuggee: finished
# Debuggee PASSED with exit code: 95
# TEST PASSED
----------System.err:(0/0)----------
----------rerun:(38/8930)*----------