Greetings. I am currently in the process of moving a cluster I administer from NFS/TCP to NFS/RDMA, and am running into a number of issues I'd like some assistance with. Googling these doesn't help.
For background on what caused me to move to NFS/TCP in the first place, please see the thread that starts at http://lkml.org/lkml/2010/8/23/204 The main reason I'm moving away from NFS/TCP is that something happened in the later kernels that reduces its resilience. Specifically, the client now permanently loses contact with the server whenever the latter fails to allocate an RPC sk_buff due to memory fragmentation. Restarting the server's nfsd's fixes this problem, at least temporarily. I haven't nailed down when this started happening (somewhere since 2.6.38), nor am I inclined to do so. This new experience (for me) with NFS/TCP has conclusively shown me that it is much more responsive with smaller IPoIB MTU's. Thus I will instead be reducing that MTU from its connected mode maximum of 65520, perhaps all the way down to datagram mode's 2044, to completely factor out memory fragmentation effects. More on that below. In moving to NFS/RDMA and reducing the IPoIB MTU, I have seen the following behaviours. -- 1) Random client-side BUG()'outs. In fact, these never finish producing a complete stack trace. I've tracked this down to duplicate replies being encountered by rpcrdma_reply_handler() in net/sunrpc/xprtrdma/rpc_rdma.c. Frankly I don't see why rpcrdma_reply_handler() should BUG() out in that case given TCP's behaviour in similar situations, documented requirements for the use of BUG() & friends in the first place, and the fact that rpcrdma_reply_handler() essentially "ignores" replies for which it cannot find a corresponding request. For the past few days now, I've been running the following on some of my nodes with no ill effects. And yes, I do see the log message this produces. This changes rpcrdma_reply_handler() to treat duplicate replies in much the same way it treats replies for which it cannot find a request. diff -adNpru linux-3.1.6/net/sunrpc/xprtrdma/rpc_rdma.c devel-3.1.6/net/sunrpc/xprtrdma/rpc_rdma.c --- linux-3.1.6/net/sunrpc/xprtrdma/rpc_rdma.c 2011-12-21 14:00:46.000000000 -0700 +++ devel-3.1.6/net/sunrpc/xprtrdma/rpc_rdma.c 2011-12-29 07:25:59.000000000 -0700 @@ -776,7 +776,13 @@ repost: " RPC request 0x%p xid 0x%08x\n", __func__, rep, req, rqst, headerp->rm_xid); - BUG_ON(!req || req->rl_reply); + /* req cannot be NULL here */ + if (req->rl_reply) { + spin_unlock(&xprt->transport_lock); + printk(KERN_NOTICE "RPC: %s: duplicate replies to request 0x%p: " + "0x%p and 0x%p\n", __func__, req, req->rl_reply, rep); + goto repost; + } /* from here on, the reply is no longer an orphan */ req->rl_reply = rep; This would also apply, modulo patch fuzz, all the way back to 2.6.24. -- 2) Still client-side, I'm seeing a lot of these sequences ... rpcrdma: connection to 10.0.6.1:20049 on mthca0, memreg 6 slots 32 ird 4 rpcrdma: connection to 10.0.6.1:20049 closed (-103) 103 is ECONNABORTED. memreg 6 is RPCRDMA_ALLPHYSICAL, so I'm assuming my Mellanox adapters don't support the default RPCRDMA_FRMR (memreg 5). I've traced these aborted connections to IB_CM_DREP_RECEIVED events being received by cma_ib_handler() in drivers/infiniband/core/cma.c, but can go no further given my limited understanding of what this code is supposed to do. I am guessing though, that these would presumably disappear when switching back to datagram mode (cm == connected mode). These messages don't appear to affect anything (the client simply reconnects and I've seen no data corruption), but it would still be nice to know what's going on here. -- 3) isn't related to NFS/RDMA per se, but to my attempts at reducing the IPoIB MTU. Whenever I do so on the fly across the cluster, some but not all, IPoIB traffic simply times out. Even, in some cases, TCP connections accept()'ed after the MTU reduction. Oddly, neither NFS/TCP nor NFS/RDMA seem affected, but other things (MPI apps, torque, etc.) are, whether started before or after the change. So, something, somewhere, remembers the previous (larger) MTU (opensm?). It seems that the only way to clear this "memory" is to reboot the entire cluster, something I'd rather avoid if possible. -- 4) Lastly, I would like to ask for a better understanding of the relationship, if any, between NFS/RDMA and the IPoIB MTU, and between NFS/RDMA and [rw]size NFS mount parameters. What effect do these have on NFS/RDMA? -- Please CC me on any comments/flames about any of the above as I am not subscribed to this list. Thanks. Marc. +----------------------------------+----------------------------------+ | Marc Aurele La France | work: 1-780-492-9310 | | Academic Information and | fax: 1-780-492-1729 | | Communications Technologies | email: [email protected] | | 352 General Services Building +----------------------------------+ | University of Alberta | | | Edmonton, Alberta | Standard disclaimers apply | | T6G 2H1 | | | CANADA | | +----------------------------------+----------------------------------+ -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
