From: Chuck Lever <chuck.lever@oracle.com>
Date: Fri, 4 May 2018 15:35:57 -0400
Subject: xprtrdma: Wait on empty sendctx queue
Patch-mainline: v4.18-rc1
Git-commit: 2fad659209d5b1dbaa1f58606372571c61fc8cbd
References: bsc#1103992 FATE#326009

Currently, when the sendctx queue is exhausted during marshaling, the
RPC/RDMA transport places the RPC task on the delayq, which forces a
wait for HZ >> 2 before the marshal and send is retried.

With this change, the transport now places such an RPC task on the
pending queue, and wakes it just as soon as more sendctxs become
available. This typically takes less than a millisecond, and the
write_space waking mechanism is less deadlock-prone.

Moreover, the waiting RPC task is holding the transport's write
lock, which blocks the transport from sending RPCs. Therefore faster
recovery from sendctx queue exhaustion is desirable.

Cf. commit 5804891455d5 ("xprtrdma: ->send_request returns -EAGAIN
when there are no free MRs").

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 net/sunrpc/xprtrdma/rpc_rdma.c  |    2 +-
 net/sunrpc/xprtrdma/verbs.c     |    8 +++++++-
 net/sunrpc/xprtrdma/xprt_rdma.h |    6 ++++++
 3 files changed, 14 insertions(+), 2 deletions(-)

--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -698,7 +698,7 @@ rpcrdma_prepare_send_sges(struct rpcrdma
 {
 	req->rl_sendctx = rpcrdma_sendctx_get_locked(&r_xprt->rx_buf);
 	if (!req->rl_sendctx)
-		return -ENOBUFS;
+		return -EAGAIN;
 	req->rl_sendctx->sc_wr.num_sge = 0;
 	req->rl_sendctx->sc_unmap_count = 0;
 	req->rl_sendctx->sc_req = req;
--- a/net/sunrpc/xprtrdma/verbs.c
+++ b/net/sunrpc/xprtrdma/verbs.c
@@ -879,6 +879,7 @@ static int rpcrdma_sendctxs_create(struc
 		sc->sc_xprt = r_xprt;
 		buf->rb_sc_ctxs[i] = sc;
 	}
+	buf->rb_flags = 0;
 
 	return 0;
 
@@ -936,7 +937,7 @@ out_emptyq:
 	 * completions recently. This is a sign the Send Queue is
 	 * backing up. Cause the caller to pause and try again.
 	 */
-	dprintk("RPC:       %s: empty sendctx queue\n", __func__);
+	set_bit(RPCRDMA_BUF_F_EMPTY_SCQ, &buf->rb_flags);
 	r_xprt = container_of(buf, struct rpcrdma_xprt, rx_buf);
 	r_xprt->rx_stats.empty_sendctx_q++;
 	return NULL;
@@ -971,6 +972,11 @@ rpcrdma_sendctx_put_locked(struct rpcrdm
 
 	/* Paired with READ_ONCE */
 	smp_store_release(&buf->rb_sc_tail, next_tail);
+
+	if (test_and_clear_bit(RPCRDMA_BUF_F_EMPTY_SCQ, &buf->rb_flags)) {
+		smp_mb__after_atomic();
+		xprt_write_space(&sc->sc_xprt->rx_xprt);
+	}
 }
 
 static void
--- a/net/sunrpc/xprtrdma/xprt_rdma.h
+++ b/net/sunrpc/xprtrdma/xprt_rdma.h
@@ -400,6 +400,7 @@ struct rpcrdma_buffer {
 	spinlock_t		rb_lock;	/* protect buf lists */
 	struct list_head	rb_send_bufs;
 	struct list_head	rb_recv_bufs;
+	unsigned long		rb_flags;
 	u32			rb_max_requests;
 	u32			rb_credits;	/* most recent credit grant */
 	int			rb_posted_receives;
@@ -417,6 +418,11 @@ struct rpcrdma_buffer {
 };
 #define rdmab_to_ia(b) (&container_of((b), struct rpcrdma_xprt, rx_buf)->rx_ia)
 
+/* rb_flags */
+enum {
+	RPCRDMA_BUF_F_EMPTY_SCQ = 0,
+};
+
 /*
  * Internal structure for transport instance creation. This
  * exists primarily for modularity.