[PATCH v2 18/18] sunrpc: Skip xpt_reserved accounting for non-UDP transports

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

The xpt_reserved counter exists for UDP socket-buffer back-pressure:
svc_udp_has_wspace() is the only has_wspace implementation that
consults it. Neither svc_tcp_has_wspace() nor svc_rdma_has_wspace()
read this counter.

On TCP and RDMA transports, svc_reserve() fires twice per RPC, each
time executing an atomic_sub on xpt_reserved, smp_mb(), and a
svc_xprt_enqueue() attempt that bails on XPT_BUSY. At 257K ops/sec
over NFS/RDMA, this is measurable dead overhead.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 include/linux/sunrpc/svc_xprt.h |  2 ++
 net/sunrpc/svc_xprt.c           | 22 +++++++++++++---------
 net/sunrpc/svcsock.c            |  1 +
 3 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/include/linux/sunrpc/svc_xprt.h b/include/linux/sunrpc/svc_xprt.h
index da2a2531e110..077cec38ed8d 100644
--- a/include/linux/sunrpc/svc_xprt.h
+++ b/include/linux/sunrpc/svc_xprt.h
@@ -37,6 +37,8 @@ struct svc_xprt_class {
 	struct list_head	xcl_list;
 	u32			xcl_max_payload;
 	int			xcl_ident;
+	u32			xcl_flags;
+#define SVC_XPRT_FLAG_WSPACE_RESERVE	BIT(0)
 };
 
 /*
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 36c8437cfd8d..94d21b68c1f8 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -468,11 +468,11 @@ static bool svc_xprt_ready(struct svc_xprt *xprt)
 
 	/*
 	 * If another cpu has recently updated xpt_flags,
-	 * sk_sock->flags, xpt_reserved, or xpt_nr_rqsts, we need to
-	 * know about it; otherwise it's possible that both that cpu and
-	 * this one could call svc_xprt_enqueue() without either
-	 * svc_xprt_enqueue() recognizing that the conditions below
-	 * are satisfied, and we could stall indefinitely:
+	 * sk_sock->flags, xpt_reserved (UDP only), or xpt_nr_rqsts,
+	 * we need to know about it; otherwise it's possible that both
+	 * that cpu and this one could call svc_xprt_enqueue() without
+	 * either svc_xprt_enqueue() recognizing that the conditions
+	 * below are satisfied, and we could stall indefinitely:
 	 */
 	smp_rmb();
 	xpt_flags = READ_ONCE(xprt->xpt_flags);
@@ -552,10 +552,13 @@ void svc_reserve(struct svc_rqst *rqstp, int space)
 	space += rqstp->rq_res.head[0].iov_len;
 
 	if (xprt && space < rqstp->rq_reserved) {
-		atomic_sub((rqstp->rq_reserved - space),
-			   &xprt->xpt_reserved);
+		if (xprt->xpt_class->xcl_flags & SVC_XPRT_FLAG_WSPACE_RESERVE) {
+			atomic_sub((rqstp->rq_reserved - space),
+				   &xprt->xpt_reserved);
+		}
 		rqstp->rq_reserved = space;
-		svc_xprt_resource_released(xprt);
+		if (xprt->xpt_class->xcl_flags & SVC_XPRT_FLAG_WSPACE_RESERVE)
+			svc_xprt_resource_released(xprt);
 	}
 }
 EXPORT_SYMBOL_GPL(svc_reserve);
@@ -834,7 +837,8 @@ static void svc_handle_xprt(struct svc_rqst *rqstp, struct svc_xprt *xprt)
 		else
 			len = xprt->xpt_ops->xpo_recvfrom(rqstp);
 		rqstp->rq_reserved = serv->sv_max_mesg;
-		atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
+		if (xprt->xpt_class->xcl_flags & SVC_XPRT_FLAG_WSPACE_RESERVE)
+			atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
 		if (len <= 0)
 			goto out;
 
diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index f28c6076f7e8..ce840f8e86c6 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -828,6 +828,7 @@ static struct svc_xprt_class svc_udp_class = {
 	.xcl_ops = &svc_udp_ops,
 	.xcl_max_payload = RPCSVC_MAXPAYLOAD_UDP,
 	.xcl_ident = XPRT_TRANSPORT_UDP,
+	.xcl_flags = SVC_XPRT_FLAG_WSPACE_RESERVE,
 };
 
 static void svc_udp_init(struct svc_sock *svsk, struct svc_serv *serv)
-- 
2.53.0