Details
Major Description
A LNet router crash regularly with OOM on a compute partition at the CEA.
Each time, the router complains about a compute node (with RDMA timeout) and then crash with OOM.
This issue seems to be linked to a defective compute rack or infiniband interface, but this should not cause the LNet router to crash.
Environment:
x32 infiniband x12 infiniband ~ x100 computes <--o2ib1--> routers <--o2ib0--> servers peer_credits = 42 discovery = 0 health_sensitivity = 0 transaction_timeout = 50 retry_count = 0 router RAM amount : 48GB
Kdumps information:
On the peer interface (lnet_peer_ni) to the faulty compute:
tx credits: ~ -4500
I read the msg tx queue (lpni_txq) and sort the messages by NID sources: for 69 NIDs I count 42 messages (peer_credits value) blocked in the tx queue.
I found the peer interface with a server NID that have 42 msg blocked on tx:
peer buffer credit: ~ -17000
On the peer router queue (lpni_rtrq), messages seems to be linked to different kib_conn (kib_rx.rx_conn) every 42 messages.
These connections are in disconnected state, with ibc_list and ibc_sched_list not linked (poison value inside). But qp and cq are not freed.
A QP take 512 pages and a CQ take 256 pages, ~ 3 MB per connection.
So it seems to be a connections leak.
Analyze
Here what I understood with the lnet/ko2iblnd sources:
1. Compute node have an issue and do not answer (or partially) to the router.
2. Messages from the servers to the compute node are queued and the tx peer credits is negative.
3. When a server peer interface have more than 42 messages blocked on tx, peer_buffer_credits is negative (by default, peer_credits == peer_buffer_credits). In that case, new message from server are queued in lpni_rtrq.
4. After that, the server is not able to send any messages to the router because peer_buffer_credits < 0. All messages from the server sent to the router timeout (RDMA timeout).
5. The server disconnects/reconnects to the routers and cleans its tx credits and resend its messages.
6. On the router, the old connection is set to disconnect but not freed because old Rx messages are not cleaned and still reference the old connection.
Can someone help me with this ?
I am not used to debug LNet/ko2iblnd.