SUNRPC: Reduce thread wake-up rate when receiving large RPC messages

With large NFS WRITE requests on TCP, I measured 5-10 thread wake-
ups to receive each request. This is because the socket layer
calls ->sk_data_ready() frequently, and each call triggers a
thread wake-up. Each recvmsg() seems to pull in less than 100KB.

Have the socket layer hold ->sk_data_ready() calls until the full
incoming message has arrived to reduce the wake-up rate.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svcsock.c

@@ -1089,6 +1089,9 @@ static void svc_tcp_fragment_received(struct svc_sock *svsk)
 	/* If we have more data, signal svc_xprt_enqueue() to try again */
 	svsk->sk_tcplen = 0;
 	svsk->sk_marker = xdr_zero;
+
+	smp_wmb();
+	tcp_set_rcvlowat(svsk->sk_sk, 1);
 }
 
 /**
@@ -1178,10 +1181,17 @@ static int svc_tcp_recvfrom(struct svc_rqst *rqstp)
 		goto err_delete;
 	if (len == want)
 		svc_tcp_fragment_received(svsk);
-	else
+	else {
+		/* Avoid more ->sk_data_ready() calls until the rest
+		 * of the message has arrived. This reduces service
+		 * thread wake-ups on large incoming messages. */
+		tcp_set_rcvlowat(svsk->sk_sk,
+				 svc_sock_reclen(svsk) - svsk->sk_tcplen);
+
 		trace_svcsock_tcp_recv_short(&svsk->sk_xprt,
 				svc_sock_reclen(svsk),
 				svsk->sk_tcplen - sizeof(rpc_fraghdr));
+	}
 	goto err_noclose;
 error:
 	if (len != -EAGAIN)