[PATCH v3 7/7] xprtrdma: Post receive buffers after RPC completion

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

rpcrdma_post_recvs() runs in CQ poll context and its cost
falls on the latency-critical path between polling a Receive
completion and waking the RPC consumer. Every cycle spent
refilling the Receive Queue delays delivery of the reply to
the NFS layer.

Move the rpcrdma_post_recvs() call in rpcrdma_reply_handler()
to after the RPC has been decoded and completed. The larger
batch size from the preceding patch provides sufficient
Receive Queue headroom to absorb the brief delay before
buffers are replenished.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/xprtrdma/rpc_rdma.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
index a77e7e48aab2..0e0f21974710 100644
--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -1422,7 +1422,6 @@ void rpcrdma_reply_handler(struct rpcrdma_rep *rep)
 		credits = 1;	/* don't deadlock */
 	else if (credits > r_xprt->rx_ep->re_max_requests)
 		credits = r_xprt->rx_ep->re_max_requests;
-	rpcrdma_post_recvs(r_xprt, credits + (buf->rb_bc_srv_max_requests << 1));
 	if (buf->rb_credits != credits)
 		rpcrdma_update_cwnd(r_xprt, credits);
 
@@ -1441,15 +1440,20 @@ void rpcrdma_reply_handler(struct rpcrdma_rep *rep)
 		/* LocalInv completion will complete the RPC */
 	else
 		kref_put(&req->rl_kref, rpcrdma_reply_done);
-	return;
 
-out_badversion:
-	trace_xprtrdma_reply_vers_err(rep);
-	goto out;
+out_post:
+	rpcrdma_post_recvs(r_xprt,
+			   credits + (buf->rb_bc_srv_max_requests << 1));
+	return;
 
 out_norqst:
 	spin_unlock(&xprt->queue_lock);
 	trace_xprtrdma_reply_rqst_err(rep);
+	rpcrdma_rep_put(buf, rep);
+	goto out_post;
+
+out_badversion:
+	trace_xprtrdma_reply_vers_err(rep);
 	goto out;
 
 out_shortreply:
-- 
2.53.0