svcrdma: Persistently allocate and DMA-map Send buffers

While sending each RPC Reply, svc_rdma_sendto allocates and DMA-
maps a separate buffer where the RPC/RDMA transport header is
constructed. The buffer is unmapped and released in the Send
completion handler. This is significant per-RPC overhead,
especially for small RPCs.

Instead, allocate and DMA-map a buffer, and cache it in each
svc_rdma_send_ctxt. This buffer and its mapping can be re-used
for each RPC, saving the cost of memory allocation and DMA
mapping.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

1 files changed, 17 insertions, 34 deletions

diff --git a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
index 40f5e4afbcc8..343e7add672c 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
@@ -115,43 +115,21 @@ out_notfound:
  * the adapter has a small maximum SQ depth.
  */
 static int svc_rdma_bc_sendto(struct svcxprt_rdma *rdma,
-			      struct rpc_rqst *rqst)
+			      struct rpc_rqst *rqst,
+			      struct svc_rdma_send_ctxt *ctxt)
 {
-	struct svc_rdma_send_ctxt *ctxt;
 	int ret;
 
-	ctxt = svc_rdma_send_ctxt_get(rdma);
-	if (!ctxt) {
-		ret = -ENOMEM;
-		goto out_err;
-	}
-
-	/* rpcrdma_bc_send_request builds the transport header and
-	 * the backchannel RPC message in the same buffer. Thus only
-	 * one SGE is needed to send both.
-	 */
-	ret = svc_rdma_map_reply_hdr(rdma, ctxt, rqst->rq_buffer,
-				     rqst->rq_snd_buf.len);
+	ret = svc_rdma_map_reply_msg(rdma, ctxt, &rqst->rq_snd_buf, NULL);
 	if (ret < 0)
-		goto out_err;
+		return -EIO;
 
 	/* Bump page refcnt so Send completion doesn't release
 	 * the rq_buffer before all retransmits are complete.
 	 */
 	get_page(virt_to_page(rqst->rq_buffer));
 	ctxt->sc_send_wr.opcode = IB_WR_SEND;
-	ret = svc_rdma_send(rdma, &ctxt->sc_send_wr);
-	if (ret)
-		goto out_unmap;
-
-out_err:
-	dprintk("svcrdma: %s returns %d\n", __func__, ret);
-	return ret;
-
-out_unmap:
-	svc_rdma_send_ctxt_put(rdma, ctxt);
-	ret = -EIO;
-	goto out_err;
+	return svc_rdma_send(rdma, &ctxt->sc_send_wr);
 }
 
 /* Server-side transport endpoint wants a whole page for its send
@@ -198,13 +176,15 @@ rpcrdma_bc_send_request(struct svcxprt_rdma *rdma, struct rpc_rqst *rqst)
 {
 	struct rpc_xprt *xprt = rqst->rq_xprt;
 	struct rpcrdma_xprt *r_xprt = rpcx_to_rdmax(xprt);
+	struct svc_rdma_send_ctxt *ctxt;
 	__be32 *p;
 	int rc;
 
-	/* Space in the send buffer for an RPC/RDMA header is reserved
-	 * via xprt->tsh_size.
-	 */
-	p = rqst->rq_buffer;
+	ctxt = svc_rdma_send_ctxt_get(rdma);
+	if (!ctxt)
+		goto drop_connection;
+
+	p = ctxt->sc_xprt_buf;
 	*p++ = rqst->rq_xid;
 	*p++ = rpcrdma_version;
 	*p++ = cpu_to_be32(r_xprt->rx_buf.rb_bc_max_requests);
@@ -212,14 +192,17 @@ rpcrdma_bc_send_request(struct svcxprt_rdma *rdma, struct rpc_rqst *rqst)
 	*p++ = xdr_zero;
 	*p++ = xdr_zero;
 	*p   = xdr_zero;
+	svc_rdma_sync_reply_hdr(rdma, ctxt, RPCRDMA_HDRLEN_MIN);
 
 #ifdef SVCRDMA_BACKCHANNEL_DEBUG
 	pr_info("%s: %*ph\n", __func__, 64, rqst->rq_buffer);
 #endif
 
-	rc = svc_rdma_bc_sendto(rdma, rqst);
-	if (rc)
+	rc = svc_rdma_bc_sendto(rdma, rqst, ctxt);
+	if (rc) {
+		svc_rdma_send_ctxt_put(rdma, ctxt);
 		goto drop_connection;
+	}
 	return rc;
 
 drop_connection:
@@ -327,7 +310,7 @@ xprt_setup_rdma_bc(struct xprt_create *args)
 	xprt->idle_timeout = RPCRDMA_IDLE_DISC_TO;
 
 	xprt->prot = XPRT_TRANSPORT_BC_RDMA;
-	xprt->tsh_size = RPCRDMA_HDRLEN_MIN / sizeof(__be32);
+	xprt->tsh_size = 0;
 	xprt->ops = &xprt_rdma_bc_procs;
 
 	memcpy(&xprt->addr, args->dstaddr, args->addrlen);