xprtrdma: Limit work done by completion handler

Sagi Grimberg <sagig@dev.mellanox.co.il> points out that a steady
stream of CQ events could starve other work because of the boundless
loop pooling in rpcrdma_{send,recv}_poll().

Instead of a (potentially infinite) while loop, return after
collecting a budgeted number of completions.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: Sagi Grimberg <sagig@dev.mellanox.co.il>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/verbs.c

@@ -165,8 +165,9 @@ static int
 rpcrdma_sendcq_poll(struct ib_cq *cq, struct rpcrdma_ep *ep)
 {
 	struct ib_wc *wcs;
-	int count, rc;
+	int budget, count, rc;
 
+	budget = RPCRDMA_WC_BUDGET / RPCRDMA_POLLSIZE;
 	do {
 		wcs = ep->rep_send_wcs;
 
@@ -177,7 +178,7 @@ rpcrdma_sendcq_poll(struct ib_cq *cq, struct rpcrdma_ep *ep)
 		count = rc;
 		while (count-- > 0)
 			rpcrdma_sendcq_process_wc(wcs++);
-	} while (rc == RPCRDMA_POLLSIZE);
+	} while (rc == RPCRDMA_POLLSIZE && --budget);
 	return 0;
 }
 
@@ -254,8 +255,9 @@ static int
 rpcrdma_recvcq_poll(struct ib_cq *cq, struct rpcrdma_ep *ep)
 {
 	struct ib_wc *wcs;
-	int count, rc;
+	int budget, count, rc;
 
+	budget = RPCRDMA_WC_BUDGET / RPCRDMA_POLLSIZE;
 	do {
 		wcs = ep->rep_recv_wcs;
 
@@ -266,7 +268,7 @@ rpcrdma_recvcq_poll(struct ib_cq *cq, struct rpcrdma_ep *ep)
 		count = rc;
 		while (count-- > 0)
 			rpcrdma_recvcq_process_wc(wcs++);
-	} while (rc == RPCRDMA_POLLSIZE);
+	} while (rc == RPCRDMA_POLLSIZE && --budget);
 	return 0;
 }
 

net/sunrpc/xprtrdma/xprt_rdma.h

@@ -74,6 +74,7 @@ struct rpcrdma_ia {
  * RDMA Endpoint -- one per transport instance
  */
 
+#define RPCRDMA_WC_BUDGET	(128)
 #define RPCRDMA_POLLSIZE	(16)
 
 struct rpcrdma_ep {