[PATCH v1] xprtrdma: Spread reply processing over more CPUs

[Chuck Lever <chuck.lever-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

Commit d8f532d20ee4 ("xprtrdma: Invoke rpcrdma_reply_handler
directly from RECV completion") introduced a performance regression
for NFS I/O small enough to not need memory registration. In multi-
threaded benchmarks that generate primarily small I/O requests,
IOPS throughput is reduced by nearly a third. This patch restores
the previous level of throughput.

Because workqueues are typically BOUND (in particular ib_comp_wq,
nfsiod_workqueue, and rpciod_workqueue), NFS/RDMA workloads tend
to aggregate on the CPU that is handling Receive completions.

The usual approach to addressing this problem is to create a QP
and CQ for each CPU, and then schedule transactions on the QP
for the CPU where you want the transaction to complete. The
transaction then does not require an extra context switch during
completion to end up on the same CPU where the transaction was
started.

This approach doesn't work for the Linux NFS/RDMA client because
currently the Linux NFS client does not support multiple connections
per client-server pair, and the RDMA core API does not make it
straightforward for ULPs to determine which CPU is responsible for
handling Receive completions for a CQ.

So for the moment, record the CPU number in the rpcrdma_req before
the transport sends each RPC Call. Then during Receive completion,
queue the RPC completion on that same CPU.

Additionally, move all RPC completion processing to the deferred
handler so that even RPCs with simple small replies complete on
the CPU that sent the corresponding RPC Call.

Fixes: d8f532d20ee4 ("xprtrdma: Invoke rpcrdma_reply_handler ...")
Signed-off-by: Chuck Lever <chuck.lever-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
---

Hi Anna-

Could you pull this into v4.15-rc ?

 net/sunrpc/xprtrdma/rpc_rdma.c  |    7 +------
 net/sunrpc/xprtrdma/transport.c |    2 ++
 net/sunrpc/xprtrdma/verbs.c     |    2 +-
 net/sunrpc/xprtrdma/xprt_rdma.h |    1 +
 4 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
index e33591f..5e846f7 100644
--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -1367,12 +1367,7 @@ void rpcrdma_reply_handler(struct rpcrdma_rep *rep)
 	clear_bit(RPCRDMA_REQ_F_PENDING, &req->rl_flags);
 
 	trace_xprtrdma_reply(rqst->rq_task, rep, req, credits);
-
-	if (list_empty(&req->rl_registered) &&
-	    !test_bit(RPCRDMA_REQ_F_TX_RESOURCES, &req->rl_flags))
-		rpcrdma_complete_rqst(rep);
-	else
-		queue_work(rpcrdma_receive_wq, &rep->rr_work);
+	queue_work_on(req->rl_cpu, rpcrdma_receive_wq, &rep->rr_work);
 	return;
 
 out_badstatus:
diff --git a/net/sunrpc/xprtrdma/transport.c b/net/sunrpc/xprtrdma/transport.c
index 8f677ab..3543c13 100644
--- a/net/sunrpc/xprtrdma/transport.c
+++ b/net/sunrpc/xprtrdma/transport.c
@@ -52,6 +52,7 @@
 #include <linux/slab.h>
 #include <linux/seq_file.h>
 #include <linux/sunrpc/addr.h>
+#include <linux/smp.h>
 
 #include "xprt_rdma.h"
 
@@ -654,6 +655,7 @@
 		task->tk_pid, __func__, rqst->rq_callsize,
 		rqst->rq_rcvsize, req);
 
+	req->rl_cpu = smp_processor_id();
 	req->rl_connect_cookie = 0;	/* our reserved value */
 	rpcrdma_set_xprtdata(rqst, req);
 	rqst->rq_buffer = req->rl_sendbuf->rg_base;
diff --git a/net/sunrpc/xprtrdma/verbs.c b/net/sunrpc/xprtrdma/verbs.c
index 8c169f1..495b655 100644
--- a/net/sunrpc/xprtrdma/verbs.c
+++ b/net/sunrpc/xprtrdma/verbs.c
@@ -83,7 +83,7 @@
 	struct workqueue_struct *recv_wq;
 
 	recv_wq = alloc_workqueue("xprtrdma_receive",
-				  WQ_MEM_RECLAIM | WQ_UNBOUND | WQ_HIGHPRI,
+				  WQ_MEM_RECLAIM | WQ_HIGHPRI,
 				  0);
 	if (!recv_wq)
 		return -ENOMEM;
diff --git a/net/sunrpc/xprtrdma/xprt_rdma.h b/net/sunrpc/xprtrdma/xprt_rdma.h
index 430a6de..69883a9 100644
--- a/net/sunrpc/xprtrdma/xprt_rdma.h
+++ b/net/sunrpc/xprtrdma/xprt_rdma.h
@@ -334,6 +334,7 @@ enum {
 struct rpcrdma_buffer;
 struct rpcrdma_req {
 	struct list_head	rl_list;
+	int			rl_cpu;
 	unsigned int		rl_connect_cookie;
 	struct rpcrdma_buffer	*rl_buffer;
 	struct rpcrdma_rep	*rl_reply;

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