SUNRPC: Use TCP_CORK to optimise send performance on the server

Use a counter to keep track of how many requests are queued behind the
xprt->xpt_mutex, and keep TCP_CORK set until the queue is empty.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Link: https://lore.kernel.org/linux-nfs/20210213202532.23146-1-trondmy@kernel.org/T/#uSigned-off-by: Chuck Lever <chuck.lever@oracle.com>

include/linux/sunrpc/svcsock.h

@@ -35,6 +35,8 @@ struct svc_sock {
 	/* Total length of the data (not including fragment headers)
 	 * received so far in the fragments making up this rpc: */
 	u32			sk_datalen;
+	/* Number of queued send requests */
+	atomic_t		sk_sendqlen;
 
 	struct page *		sk_pages[RPCSVC_MAXPAGES];	/* received data */
 };

net/sunrpc/svcsock.c

@@ -1171,18 +1171,23 @@ static int svc_tcp_sendto(struct svc_rqst *rqstp)
 
 	svc_tcp_release_rqst(rqstp);
 
+	atomic_inc(&svsk->sk_sendqlen);
 	mutex_lock(&xprt->xpt_mutex);
 	if (svc_xprt_is_dead(xprt))
 		goto out_notconn;
+	tcp_sock_set_cork(svsk->sk_sk, true);
 	err = svc_tcp_sendmsg(svsk->sk_sock, &msg, xdr, marker, &sent);
 	xdr_free_bvec(xdr);
 	trace_svcsock_tcp_send(xprt, err < 0 ? err : sent);
 	if (err < 0 || sent != (xdr->len + sizeof(marker)))
 		goto out_close;
+	if (atomic_dec_and_test(&svsk->sk_sendqlen))
+		tcp_sock_set_cork(svsk->sk_sk, false);
 	mutex_unlock(&xprt->xpt_mutex);
 	return sent;
 
 out_notconn:
+	atomic_dec(&svsk->sk_sendqlen);
 	mutex_unlock(&xprt->xpt_mutex);
 	return -ENOTCONN;
 out_close:
@@ -1192,6 +1197,7 @@ static int svc_tcp_sendto(struct svc_rqst *rqstp)
 		  (err < 0) ? err : sent, xdr->len);
 	set_bit(XPT_CLOSE, &xprt->xpt_flags);
 	svc_xprt_enqueue(xprt);
+	atomic_dec(&svsk->sk_sendqlen);
 	mutex_unlock(&xprt->xpt_mutex);
 	return -EAGAIN;
 }
@@ -1261,7 +1267,7 @@ static void svc_tcp_init(struct svc_sock *svsk, struct svc_serv *serv)
 		svsk->sk_datalen = 0;
 		memset(&svsk->sk_pages[0], 0, sizeof(svsk->sk_pages));
 
-		tcp_sk(sk)->nonagle |= TCP_NAGLE_OFF;
+		tcp_sock_set_nodelay(sk);
 
 		set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
 		switch (sk->sk_state) {