xprtrdma: More Send completion batching

Instead of using a fixed number, allow the amount of Send completion
batching to vary based on the client's maximum credit limit.

- A larger default gives a small boost to IOPS throughput

- Reducing it based on max_requests gives a safe result when the
  max credit limit is cranked down (eg. when the device has a small
  max_qp_wr).
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/verbs.c

@@ -521,9 +521,7 @@ rpcrdma_ep_create(struct rpcrdma_ep *ep, struct rpcrdma_ia *ia,
 		ep->rep_attr.cap.max_send_sge,
 		ep->rep_attr.cap.max_recv_sge);
 
-	/* set trigger for requesting send completion */
-	ep->rep_send_batch = min_t(unsigned int, RPCRDMA_MAX_SEND_BATCH,
-				   cdata->max_requests >> 2);
+	ep->rep_send_batch = cdata->max_requests >> 3;
 	ep->rep_send_count = ep->rep_send_batch;
 	init_waitqueue_head(&ep->rep_connect_wait);
 	ep->rep_receive_count = 0;

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -232,16 +232,6 @@ struct rpcrdma_sendctx {
 	struct ib_sge		sc_sges[];
 };
 
-/* Limit the number of SGEs that can be unmapped during one
- * Send completion. This caps the amount of work a single
- * completion can do before returning to the provider.
- *
- * Setting this to zero disables Send completion batching.
- */
-enum {
-	RPCRDMA_MAX_SEND_BATCH = 7,
-};
-
 /*
  * struct rpcrdma_mr - external memory region metadata
  *