[PATCH RFC] SUNRPC: Use zero-copy to perform socket send operations

[Chuck Lever <chuck.lever@oracle.com>]

Daire Byrne reports a ~50% aggregrate throughput regression on his
Linux NFS server after commit da1661b93bf4 ("SUNRPC: Teach server to
use xprt_sock_sendmsg for socket sends"), which replaced
kernel_send_page() calls in NFSD's socket send path with calls to
sock_sendmsg() using iov_iter.

Investigation showed that tcp_sendmsg() was not using zero-copy to
send the xdr_buf's bvec pages, but instead was relying on memcpy.

Set up the socket and each msghdr that bears bvec pages to use the
zero-copy mechanism in tcp_sendmsg.

Reported-by: Daire Byrne <daire@dneg.com>
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=209439
Fixes: da1661b93bf4 ("SUNRPC: Teach server to use xprt_sock_sendmsg for socket sends")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/socklib.c  |    5 ++++-
 net/sunrpc/svcsock.c  |    1 +
 net/sunrpc/xprtsock.c |    1 +
 3 files changed, 6 insertions(+), 1 deletion(-)

This patch does not fully resolve the issue. Daire reports high
softIRQ activity after the patch is applied, and this activity
seems to prevent full restoration of previous performance.


diff --git a/net/sunrpc/socklib.c b/net/sunrpc/socklib.c
index d52313af82bc..af47596a7bdd 100644
--- a/net/sunrpc/socklib.c
+++ b/net/sunrpc/socklib.c
@@ -226,9 +226,12 @@ static int xprt_send_pagedata(struct socket *sock, struct msghdr *msg,
 	if (err < 0)
 		return err;
 
+	msg->msg_flags |= MSG_ZEROCOPY;
 	iov_iter_bvec(&msg->msg_iter, WRITE, xdr->bvec, xdr_buf_pagecount(xdr),
 		      xdr->page_len + xdr->page_base);
-	return xprt_sendmsg(sock, msg, base + xdr->page_base);
+	err = xprt_sendmsg(sock, msg, base + xdr->page_base);
+	msg->msg_flags &= ~MSG_ZEROCOPY;
+	return err;
 }
 
 /* Common case:
diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index c2752e2b9ce3..c814b4953b15 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -1176,6 +1176,7 @@ 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));
 
+		sock_set_flag(sk, SOCK_ZEROCOPY);
 		tcp_sk(sk)->nonagle |= TCP_NAGLE_OFF;
 
 		set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
index 7090bbee0ec5..343c6396b297 100644
--- a/net/sunrpc/xprtsock.c
+++ b/net/sunrpc/xprtsock.c
@@ -2175,6 +2175,7 @@ static int xs_tcp_finish_connecting(struct rpc_xprt *xprt, struct socket *sock)
 
 		/* socket options */
 		sock_reset_flag(sk, SOCK_LINGER);
+		sock_set_flag(sk, SOCK_ZEROCOPY);
 		tcp_sk(sk)->nonagle |= TCP_NAGLE_OFF;
 
 		xprt_clear_connected(xprt);