OK -- bug can be reproduced. Here is an interesting observation: When a *second* nfs server is connected to the network , the bug seems to be suppressed, for what it's worth. This one really puzzles me. The runaway kworker appeared only after I shut down the old server.

As for debugging, the component that seems to create an insane amount of debug messages is rpcd. I am pasting snippets from nfsd's debugging-enabled and rpcd's debugging-enabled here, please let me know if you need more of the syslog file.

######### NFSD ##############
(...)
May 28 13:06:41 marcato kernel: [184158.621702] nfsd: fh_compose(exp 09:04/2 (filename redacted), ino=10619455)
May 28 13:06:41 marcato kernel: [184158.621714] nfsd: fh_compose(exp 09:04/2 (filename redacted), ino=10618649)
May 28 13:06:41 marcato kernel: [184158.621731] nfsd: fh_compose(exp 09:04/2 (filename redacted), ino=10619242)
May 28 13:06:41 marcato kernel: [184158.621742] nfsd: fh_compose(exp 09:04/2 (filename redacted), ino=10619422)
May 28 13:06:41 marcato kernel: [184158.621754] nfsd: fh_compose(exp 09:04/2 (filename redacted), ino=10619296)
May 28 13:06:41 marcato kernel: [184158.621761] nfsv4 compound op ffff880417a01080 opcnt 2 #2: 26: status 0
May 28 13:06:41 marcato kernel: [184158.621764] nfsv4 compound returned 0
May 28 13:06:41 marcato kernel: [184158.622472] nfsd_dispatch: vers 4 proc 1
May 28 13:06:41 marcato kernel: [184158.622484] nfsv4 compound op #1/2: 22 (OP_PUTFH)
May 28 13:06:41 marcato kernel: [184158.622491] nfsd: fh_verify(28: 01060001 57c17980 8845d969 d9acb495 eab15095 00a205da)
May 28 13:06:41 marcato kernel: [184158.622507] nfsv4 compound op ffff880417a01080 opcnt 2 #1: 22: status 0
May 28 13:06:41 marcato kernel: [184158.622511] nfsv4 compound op #2/2: 26 (OP_READDIR)
May 28 13:06:41 marcato kernel: [184158.622549] nfsd: fh_verify(28: 01060001 57c17980 8845d969 d9acb495 eab15095 00a205da)

######### RPCD ##############
(...)
May 28 13:07:29 marcato kernel: [184207.248774] RPC: 22256 call_connect_status (status -11)
May 28 13:07:29 marcato kernel: [184207.248777] svc: socket ffff880417284d00 TCP data ready (svsk ffff8803e3ff2000)
May 28 13:07:29 marcato kernel: [184207.248781] svc: transport ffff8803e3ff2000 served by daemon ffff8800ce6da000
May 28 13:07:29 marcato kernel: [184207.248791] RPC: 22256 call_bind (status 0)
May 28 13:07:29 marcato kernel: [184207.248793] svc: server ffff8800ce6da000, pool 0, transport ffff8803e3ff2000, inuse=2
May 28 13:07:29 marcato kernel: [184207.248795] svc: tcp_recv ffff8803e3ff2000 data 1 conn 0 close 0
May 28 13:07:29 marcato kernel: [184207.248800] svc: socket ffff8803e3ff2000 recvfrom(ffff8803e3ff22bc, 0) = 4
May 28 13:07:29 marcato kernel: [184207.248802] svc: TCP record, 188 bytes
May 28 13:07:29 marcato kernel: [184207.248806] svc: socket ffff8803e3ff2000 recvfrom(ffff880415a7c0bc, 3908) = 188
May 28 13:07:29 marcato kernel: [184207.248809] svc: TCP final record (188 bytes)
May 28 13:07:29 marcato kernel: [184207.248812] svc: got len=188
May 28 13:07:29 marcato kernel: [184207.248815] svc: svc_authenticate (1)
May 28 13:07:29 marcato kernel: [184207.248819] svc: calling dispatcher
May 28 13:07:29 marcato kernel: [184207.248826] RPC: 22256 call_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.248835] RPC: 22256 xprt_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.248844] RPC: 22256 sleep_on(queue "xprt_pending" time 4340888821)
May 28 13:07:29 marcato kernel: [184207.248851] RPC: 22256 added to queue ffff8802fd03c258 "xprt_pending"
May 28 13:07:29 marcato kernel: [184207.248862] RPC: 22256 setting alarm for 9000 ms
May 28 13:07:29 marcato kernel: [184207.248871] RPC:       xs_connect scheduled xprt ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.248885] RPC:       disconnecting xprt ffff8802fd03c000 to reuse port
May 28 13:07:29 marcato kernel: [184207.248893] RPC:       AF_UNSPEC connect return code 0
May 28 13:07:29 marcato kernel: [184207.248902] RPC:       worker connecting xprt ffff8802fd03c000 via tcp to 0.0.0.0 (port 58162)
May 28 13:07:29 marcato kernel: [184207.248921] RPC:       xs_tcp_state_change client ffff8802fd03c000...
May 28 13:07:29 marcato kernel: [184207.248922] RPC:       state 7 conn 0 dead 0 zapped 1 sk_shutdown 3
May 28 13:07:29 marcato kernel: [184207.248924] RPC:       disconnected transport ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.248925] RPC: 22256 __rpc_wake_up_task (now 4340888822)
May 28 13:07:29 marcato kernel: [184207.248926] RPC: 22256 disabling timer
May 28 13:07:29 marcato kernel: [184207.248928] RPC: 22256 removed from queue ffff8802fd03c258 "xprt_pending"
May 28 13:07:29 marcato kernel: [184207.248929] RPC:       __rpc_wake_up_task done
May 28 13:07:29 marcato kernel: [184207.248931] RPC:       ffff8802fd03c000 connect status 115 connected 0 sock state 7
May 28 13:07:29 marcato kernel: [184207.248933] RPC: 22256 __rpc_execute flags=0x681
May 28 13:07:29 marcato kernel: [184207.248934] RPC: 22256 xprt_connect_status: retrying
May 28 13:07:29 marcato kernel: [184207.248936] RPC: 22256 call_connect_status (status -11)
May 28 13:07:29 marcato kernel: [184207.248937] RPC: 22256 call_bind (status 0)
May 28 13:07:29 marcato kernel: [184207.248939] RPC: 22256 call_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.248940] RPC: 22256 xprt_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.248942] RPC: 22256 sleep_on(queue "xprt_pending" time 4340888822)
May 28 13:07:29 marcato kernel: [184207.248943] RPC: 22256 added to queue ffff8802fd03c258 "xprt_pending"
May 28 13:07:29 marcato kernel: [184207.248944] RPC: 22256 setting alarm for 9000 ms
May 28 13:07:29 marcato kernel: [184207.248946] RPC:       xs_connect scheduled xprt ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.248948] RPC:       disconnecting xprt ffff8802fd03c000 to reuse port
May 28 13:07:29 marcato kernel: [184207.248949] RPC:       AF_UNSPEC connect return code 0
May 28 13:07:29 marcato kernel: [184207.248951] RPC:       worker connecting xprt ffff8802fd03c000 via tcp to 0.0.0.0 (port 58162)
May 28 13:07:29 marcato kernel: [184207.248959] RPC:       xs_tcp_state_change client ffff8802fd03c000...
May 28 13:07:29 marcato kernel: [184207.248961] RPC:       state 7 conn 0 dead 0 zapped 1 sk_shutdown 3
May 28 13:07:29 marcato kernel: [184207.248962] RPC:       disconnected transport ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.248964] RPC: 22256 __rpc_wake_up_task (now 4340888822)
May 28 13:07:29 marcato kernel: [184207.248977] RPC: 22256 call_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.248979] RPC: 22256 xprt_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.248980] RPC: 22256 sleep_on(queue "xprt_pending" time 4340888822)
May 28 13:07:29 marcato kernel: [184207.248982] RPC: 22256 added to queue ffff8802fd03c258 "xprt_pending"
May 28 13:07:29 marcato kernel: [184207.248984] svc: socket ffff8803e3ff2000 sendto([ffff88011f47d000 64... ], 4092) = 4092 (addr 192.168.1.16, port=839)
May 28 13:07:29 marcato kernel: [184207.248986] svc: server ffff8800ce6da000 waiting for data (to = 900000)
May 28 13:07:29 marcato kernel: [184207.248987] RPC: 22256 setting alarm for 9000 ms
May 28 13:07:29 marcato kernel: [184207.248989] RPC:       xs_connect scheduled xprt ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.248990] RPC:       disconnecting xprt ffff8802fd03c000 to reuse port
May 28 13:07:29 marcato kernel: [184207.248991] RPC:       AF_UNSPEC connect return code 0
May 28 13:07:29 marcato kernel: [184207.248993] RPC:       worker connecting xprt ffff8802fd03c000 via tcp to 0.0.0.0 (port 58162)
May 28 13:07:29 marcato kernel: [184207.249002] RPC:       xs_tcp_state_change client ffff8802fd03c000...
May 28 13:07:29 marcato kernel: [184207.249004] RPC:       state 7 conn 0 dead 0 zapped 1 sk_shutdown 3
May 28 13:07:29 marcato kernel: [184207.249005] RPC:       disconnected transport ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.249006] RPC: 22256 __rpc_wake_up_task (now 4340888822)
May 28 13:07:29 marcato kernel: [184207.249007] RPC: 22256 disabling timer
May 28 13:07:29 marcato kernel: [184207.249008] RPC: 22256 removed from queue ffff8802fd03c258 "xprt_pending"
May 28 13:07:29 marcato kernel: [184207.249009] RPC:       __rpc_wake_up_task done
May 28 13:07:29 marcato kernel: [184207.249011] RPC:       ffff8802fd03c000 connect status 115 connected 0 sock state 7
May 28 13:07:29 marcato kernel: [184207.249012] RPC: 22256 __rpc_execute flags=0x681
May 28 13:07:29 marcato kernel: [184207.249013] RPC: 22256 xprt_connect_status: retrying
May 28 13:07:29 marcato kernel: [184207.249014] RPC: 22256 call_connect_status (status -11)
May 28 13:07:29 marcato kernel: [184207.249016] RPC: 22256 call_bind (status 0)
May 28 13:07:29 marcato kernel: [184207.249017] RPC: 22256 call_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.249018] RPC: 22256 xprt_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.249019] RPC: 22256 sleep_on(queue "xprt_pending" time 4340888822)
May 28 13:07:29 marcato kernel: [184207.249020] RPC: 22256 added to queue ffff8802fd03c258 "xprt_pending"
May 28 13:07:29 marcato kernel: [184207.249021] RPC: 22256 setting alarm for 9000 ms
May 28 13:07:29 marcato kernel: [184207.249023] RPC:       xs_connect scheduled xprt ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.249024] RPC:       disconnecting xprt ffff8802fd03c000 to reuse port
May 28 13:07:29 marcato kernel: [184207.249026] RPC:       AF_UNSPEC connect return code 0
May 28 13:07:29 marcato kernel: [184207.249027] RPC:       worker connecting xprt ffff8802fd03c000 via tcp to 0.0.0.0 (port 58162)
May 28 13:07:29 marcato kernel: [184207.249034] RPC:       xs_tcp_state_change client ffff8802fd03c000...
May 28 13:07:29 marcato kernel: [184207.249035] RPC:       state 7 conn 0 dead 0 zapped 1 sk_shutdown 3
May 28 13:07:29 marcato kernel: [184207.249036] RPC:       disconnected transport ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.249037] RPC: 22256 __rpc_wake_up_task (now 4340888822)
May 28 13:07:29 marcato kernel: [184207.249038] RPC: 22256 disabling timer
May 28 13:07:29 marcato kernel: [184207.249040] RPC: 22256 removed from queue ffff8802fd03c258 "xprt_pending"
May 28 13:07:29 marcato kernel: [184207.249041] RPC:       __rpc_wake_up_task done
May 28 13:07:29 marcato kernel: [184207.249042] RPC:       ffff8802fd03c000 connect status 115 connected 0 sock state 7
May 28 13:07:29 marcato kernel: [184207.249043] RPC: 22256 __rpc_execute flags=0x681
May 28 13:07:29 marcato kernel: [184207.249044] RPC: 22256 xprt_connect_status: retrying
May 28 13:07:29 marcato kernel: [184207.249046] RPC: 22256 call_connect_status (status -11)
May 28 13:07:29 marcato kernel: [184207.249047] RPC: 22256 call_bind (status 0)
May 28 13:07:29 marcato kernel: [184207.249048] RPC: 22256 call_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.249054] RPC: 22256 xprt_connect xprt ffff8802fd03c000 is not connected
May 28 13:07:29 marcato kernel: [184207.249062] RPC: 22256 sleep_on(queue "xprt_pending" time 4340888822)
May 28 13:07:29 marcato kernel: [184207.249070] RPC: 22256 added to queue ffff8802fd03c258 "xprt_pending"
May 28 13:07:29 marcato kernel: [184207.249078] RPC: 22256 setting alarm for 9000 ms
May 28 13:07:29 marcato kernel: [184207.249086] RPC:       xs_connect scheduled xprt ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.249093] RPC:       disconnecting xprt ffff8802fd03c000 to reuse port
May 28 13:07:29 marcato kernel: [184207.249099] RPC:       AF_UNSPEC connect return code 0
May 28 13:07:29 marcato kernel: [184207.249106] RPC:       worker connecting xprt ffff8802fd03c000 via tcp to 0.0.0.0 (port 58162)
May 28 13:07:29 marcato kernel: [184207.249119] RPC:       xs_tcp_state_change client ffff8802fd03c000...
May 28 13:07:29 marcato kernel: [184207.249127] RPC:       state 7 conn 0 dead 0 zapped 1 sk_shutdown 3
May 28 13:07:29 marcato kernel: [184207.249136] RPC:       disconnected transport ffff8802fd03c000
May 28 13:07:29 marcato kernel: [184207.249145] RPC: 22256 __rpc_wake_up_task (now 4340888822)
May 28 13:07:29 marcato kernel: [184207.249153] RPC: 22256 disabling timer
May 28 13:07:29 marcato kernel: [184207.249159] RPC: 22256 removed from queue ffff8802fd03c258 "xprt_pending"