svcrdma: Maintain a Receive water mark

Post more Receives when the number of pending Receives drops below
a water mark. The batch mechanism is disabled if the underlying
device cannot support a reasonably-sized Receive Queue.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svc_rdma.h

@@ -94,6 +94,8 @@ struct svcxprt_rdma {
 	spinlock_t	     sc_rw_ctxt_lock;
 	struct list_head     sc_rw_ctxts;
 
+	u32		     sc_pending_recvs;
+	u32		     sc_recv_batch;
 	struct list_head     sc_rq_dto_q;
 	spinlock_t	     sc_rq_dto_lock;
 	struct ib_qp         *sc_qp;

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c

@@ -285,6 +285,7 @@ static bool svc_rdma_refresh_recvs(struct svcxprt_rdma *rdma,
 		ctxt->rc_temp = temp;
 		ctxt->rc_recv_wr.next = recv_chain;
 		recv_chain = &ctxt->rc_recv_wr;
+		rdma->sc_pending_recvs++;
 	}
 	if (!recv_chain)
 		return false;
@@ -302,6 +303,8 @@ static bool svc_rdma_refresh_recvs(struct svcxprt_rdma *rdma,
 		bad_wr = bad_wr->next;
 		svc_rdma_recv_ctxt_put(rdma, ctxt);
 	}
+	/* Since we're destroying the xprt, no need to reset
+	 * sc_pending_recvs. */
 	return false;
 }
 
@@ -328,6 +331,8 @@ static void svc_rdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
 	struct ib_cqe *cqe = wc->wr_cqe;
 	struct svc_rdma_recv_ctxt *ctxt;
 
+	rdma->sc_pending_recvs--;
+
 	/* WARNING: Only wc->wr_cqe and wc->status are reliable */
 	ctxt = container_of(cqe, struct svc_rdma_recv_ctxt, rc_cqe);
 
@@ -344,8 +349,9 @@ static void svc_rdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
 	 * to reduce the likelihood of replayed requests once the
 	 * client reconnects.
 	 */
-	if (!svc_rdma_refresh_recvs(rdma, 1, false))
-		goto flushed;
+	if (rdma->sc_pending_recvs < rdma->sc_max_requests)
+		if (!svc_rdma_refresh_recvs(rdma, rdma->sc_recv_batch, false))
+			goto flushed;
 
 	/* All wc fields are now known to be valid */
 	ctxt->rc_byte_len = wc->byte_len;

net/sunrpc/xprtrdma/svc_rdma_transport.c

@@ -407,11 +407,14 @@ static struct svc_xprt *svc_rdma_accept(struct svc_xprt *xprt)
 	newxprt->sc_max_req_size = svcrdma_max_req_size;
 	newxprt->sc_max_requests = svcrdma_max_requests;
 	newxprt->sc_max_bc_requests = svcrdma_max_bc_requests;
-	rq_depth = newxprt->sc_max_requests + newxprt->sc_max_bc_requests;
+	newxprt->sc_recv_batch = RPCRDMA_MAX_RECV_BATCH;
+	rq_depth = newxprt->sc_max_requests + newxprt->sc_max_bc_requests +
+		   newxprt->sc_recv_batch;
 	if (rq_depth > dev->attrs.max_qp_wr) {
 		pr_warn("svcrdma: reducing receive depth to %d\n",
 			dev->attrs.max_qp_wr);
 		rq_depth = dev->attrs.max_qp_wr;
+		newxprt->sc_recv_batch = 1;
 		newxprt->sc_max_requests = rq_depth - 2;
 		newxprt->sc_max_bc_requests = 2;
 	}