> On Jan 12, 2017, at 12:42, Chuck Lever <chuck.lever@xxxxxxxxxx> wrote: > > >> On Jan 12, 2017, at 12:38 PM, Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote: >> >> On Fri, 2016-12-16 at 11:48 -0500, Chuck Lever wrote: >>> Current NFS clients rely on connection loss to determine when to >>> retransmit. In particular, for protocols like NFSv4, clients no >>> longer rely on RPC timeouts to drive retransmission: NFSv4 servers >>> are required to terminate a connection when they need a client to >>> retransmit pending RPCs. >>> >>> When a server is no longer reachable, either because it has crashed >>> or because the network path has broken, the server cannot actively >>> terminate a connection. Thus NFS clients depend on transport-level >>> keepalive to determine when a connection must be replaced and >>> pending RPCs retransmitted. >>> >>> However, RDMA RC connections do not have a native keepalive >>> mechanism. If an NFS/RDMA server crashes after a client has sent >>> RPCs successfully (an RC ACK has been received for all OTW RDMA >>> requests), there is no way for the client to know the connection is >>> moribund. >>> >>> In addition, new RDMA requests are subject to the RPC-over-RDMA >>> credit limit. If the client has consumed all granted credits with >>> NFS traffic, it is not allowed to send another RDMA request until >>> the server replies. Thus it has no way to send a true keepalive when >>> the workload has already consumed all credits with pending RPCs. >>> >>> To address this, we reserve one RPC-over-RDMA credit that may be >>> used only for an NFS NULL. A periodic RPC ping is done on transports >>> whenever there are outstanding RPCs. >>> >>> The purpose of this ping is to drive traffic regularly on each >>> connection to force the transport layer to disconnect it if it is no >>> longer viable. Some RDMA operations are fully offloaded to the HCA, >>> and can be successful even if the remote host has crashed. Thus an >>> operation that requires that the server is responsive is used for >>> the ping. >>> >>> This implementation re-uses existing generic RPC infrastructure to >>> form each NULL Call. An rpc_clnt context must be available to start >>> an RPC. Thus a generic keepalive mechanism is introduced so that >>> both an rpc_clnt and an rpc_xprt is available to perform the ping. >>> >>> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx> >>> --- >>> >>> Before sending this for internal testing, I'd like to hear comments >>> on this approach. It's a little more churn than I had hoped for. >>> >>> >>> fs/nfs/nfs4client.c | 1 >>> include/linux/sunrpc/clnt.h | 2 + >>> include/linux/sunrpc/sched.h | 3 + >>> include/linux/sunrpc/xprt.h | 1 >>> net/sunrpc/clnt.c | 101 >>> +++++++++++++++++++++++++++++++++++++++ >>> net/sunrpc/sched.c | 19 +++++++ >>> net/sunrpc/xprt.c | 5 ++ >>> net/sunrpc/xprtrdma/rpc_rdma.c | 4 +- >>> net/sunrpc/xprtrdma/transport.c | 13 +++++ >>> 9 files changed, 148 insertions(+), 1 deletion(-) >>> >>> diff --git a/fs/nfs/nfs4client.c b/fs/nfs/nfs4client.c >>> index 074ac71..c5f5ce8 100644 >>> --- a/fs/nfs/nfs4client.c >>> +++ b/fs/nfs/nfs4client.c >>> @@ -378,6 +378,7 @@ struct nfs_client *nfs4_init_client(struct >>> nfs_client *clp, >>> error = nfs_create_rpc_client(clp, cl_init, >>> RPC_AUTH_UNIX); >>> if (error < 0) >>> goto error; >>> + rpc_schedule_keepalive(clp->cl_rpcclient); >> >> Why do we want to enable this for non-RDMA transports? Shouldn't this >> functionality be hidden in the RDMA client code, in the same way that >> the TCP keepalive is hidden in the socket code. > > Sending a NULL request by re-using the normal RPC infrastructure > requires a struct rpc_clnt. Thus it has to be driven by an upper > layer context. > > I'm open to suggestions. > Ideally we just want this to operate when there are outstanding RPC calls waiting for a reply, am I correct? If so, perhaps we might have it triggersd by a timer that is armed in xprt->ops->send_request() and disarmed in xprt->ops->release_xprt()? It might then configure itself by looking in the xprt->recv list to find a hanging rpc_task and steal its rpc_client info. Cheers Trond -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
