[RFC PATCH 14/15] svcrdma: retry when receive queues drain transiently

[Chuck Lever <cel@kernel.org>]

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

When svc_rdma_recvfrom finds both sc_read_complete_q
and sc_rq_dto_q empty, svc_rdma_update_xpt_data clears
XPT_DATA, executes a barrier, and rechecks the queues.
If a completion arrived between the llist_del_first and
the recheck, XPT_DATA is re-set, but recvfrom returns
zero regardless. The thread then traverses the full
svc_recv cycle -- page allocation, dequeue, recvfrom,
release -- only to find the item that was already
available at the time of the recheck.

Trace data from a 256KB NFSv3 workload over RDMA shows
267,848 of 464,355 transport dequeues (57.7%) are these
empty bounces. Each bounce costs roughly 37 us. During
the READ phase, empty bounces consume 8.6% of thread
capacity and inflate inter-RPC gaps by an average of
87 us.

The calling thread holds XPT_BUSY for the duration, so
no other consumer can drain the queue between the
recheck and the retry. A retry is therefore guaranteed
to find data on its first iteration.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 net/sunrpc/xprtrdma/svc_rdma_recvfrom.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
index 2ee9819a53d7..a124c6ed057a 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
@@ -981,6 +981,7 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
 
 	rqstp->rq_xprt_ctxt = NULL;
 
+retry:
 	node = llist_del_first(&rdma_xprt->sc_read_complete_q);
 	if (node) {
 		ctxt = llist_entry(node, struct svc_rdma_recv_ctxt, rc_node);
@@ -995,8 +996,17 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
 		ctxt = llist_entry(node, struct svc_rdma_recv_ctxt, rc_node);
 	} else {
 		ctxt = NULL;
-		/* No new incoming requests, terminate the loop */
 		svc_rdma_update_xpt_data(rdma_xprt);
+		/*
+		 * A completion may have arrived between the
+		 * llist_del_first above and the queue recheck
+		 * inside svc_rdma_update_xpt_data. This thread
+		 * holds XPT_BUSY, preventing any other consumer
+		 * from draining the queue in the meantime.
+		 * Retry to avoid a full svc_recv round-trip.
+		 */
+		if (test_bit(XPT_DATA, &xprt->xpt_flags))
+			goto retry;
 	}
 
 	/* Unblock the transport for the next receive */
-- 
2.52.0