[RFC PATCH 15/15] svcrdma: clear XPT_DATA on sc_rq_dto_q consumption

[Chuck Lever <cel@kernel.org>]

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

svc_rdma_wc_receive() sets XPT_DATA when adding a
completed Receive to sc_rq_dto_q. When
svc_rdma_recvfrom() consumes the item from sc_rq_dto_q,
XPT_DATA is left set. The subsequent svc_xprt_received()
clears XPT_BUSY and re-enqueues the transport; because
stale XPT_DATA remains set, svc_xprt_enqueue() dispatches
a second thread. That thread finds both queues empty,
accomplishes nothing, and returns zero.

Trace data from a 256KB NFSv3 workload over RDMA shows
172,280 of 467,171 transport dequeues (36.9%) are these
spurious dispatches. The READ phase averages 1.99
dequeues per RPC (expected 1.0) and the WRITE phase
averages 2.77 (expected 2.0). Each wasted cycle traverses
svc_alloc_arg, svc_thread_wait_for_work,
svc_rdma_recvfrom, and svc_xprt_release before the
thread can accept new work.

Add svc_rdma_update_xpt_data() on the sc_rq_dto_q
success path, matching the existing call on the
sc_read_complete_q path added by commit 6807f36a39b7
("svcrdma: clear XPT_DATA on sc_read_complete_q
consumption"). The same barrier semantics apply: the
clear/recheck pattern in svc_rdma_update_xpt_data()
ensures a concurrent producer's llist_add + set_bit
is not lost.

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

diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
index a124c6ed057a..c56d70658068 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
@@ -994,6 +994,7 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
 	node = llist_del_first(&rdma_xprt->sc_rq_dto_q);
 	if (node) {
 		ctxt = llist_entry(node, struct svc_rdma_recv_ctxt, rc_node);
+		svc_rdma_update_xpt_data(rdma_xprt);
 	} else {
 		ctxt = NULL;
 		svc_rdma_update_xpt_data(rdma_xprt);
-- 
2.52.0