After upgrade Jenkins to 2.98.4 LTS, we found that the slaves always disconnect from server randomly, and from the server side, we checked log info with "thread terminated", how can we know why the thread is terminated? I also checked thread dump, but there are only dump for live thread but not for killed thread. The debug info for Server and Slave see below:

Debug info from Salves:

May 23, 2018 10:33:01 AM hudson.remoting.UserRequest perform
WARNING: LinkageError while performing UserRequest:UserRPCRequest(3,join)
java.lang.LinkageError: Failed to load hudson.util.ProcessTree
at hudson.remoting.RemoteClassLoader.loadClassFile(RemoteClassLoader.java:387)
at hudson.remoting.RemoteClassLoader.findClass(RemoteClassLoader.java:295)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at hudson.Proc$LocalProc.destroy(Proc.java:384)
at hudson.Proc$LocalProc.join(Proc.java:357)
at hudson.Launcher$RemoteLaunchCallable$1.join(Launcher.java:1304)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.perform(RemoteInvocationHandler.java:922)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:896)
at hudson.remoting.RemoteInvocationHandler$RPCRequest.call(RemoteInvocationHandler.java:853)
at hudson.remoting.UserRequest.perform(UserRequest.java:207)
at hudson.remoting.UserRequest.perform(UserRequest.java:53)
at hudson.remoting.Request$2.run(Request.java:358)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
at java.util.concurrent.FutureTask.run(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at hudson.remoting.Engine$1$1.run(Engine.java:98)
at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.NoClassDefFoundError: hudson/util/ProcessTreeRemoting$IProcessTree
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at hudson.remoting.RemoteClassLoader.loadClassFile(RemoteClassLoader.java:383)
... 22 more
Caused by: java.lang.ClassNotFoundException: hudson.util.ProcessTreeRemoting$IProcessTree
at java.net.URLClassLoader.findClass(Unknown Source)
at hudson.remoting.RemoteClassLoader.findClass(RemoteClassLoader.java:159)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 26 more

Debug Info from Server:
May 23, 2018 10:32:55 AM hudson.TcpSlaveAgentListener$ConnectionHandler run
INFO: Accepted JNLP4-connect connection #101,988 from /10.224.105.34:28333
May 23, 2018 10:32:57 AM jenkins.slaves.DefaultJnlpSlaveReceiver channelClosed
WARNING: IOHub#1: Worker[channel:java.nio.channels.SocketChannel[connected local=/10.224.105.17:53233 remote=avcndaevpcisrv/10.224.105.34:1655]] / Computer.threadPoolForRemoting 5173 for FTVP_CI_Slv_SSD_View terminated
java.nio.channels.ClosedChannelException
at org.jenkinsci.remoting.protocol.NetworkLayer.onRecvClosed(NetworkLayer.java:154)
at org.jenkinsci.remoting.protocol.impl.NIONetworkLayer.ready(NIONetworkLayer.java:179)
at org.jenkinsci.remoting.protocol.IOHub$OnReady.run(IOHub.java:721)
at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)