xprtrdma: Invoke rpcrdma_reply_handler directly from RECV completion

I noticed that the soft IRQ thread looked pretty busy under heavy
I/O workloads. perf suggested one area that was expensive was the
queue_work() call in rpcrdma_wc_receive. That gave me some ideas.

Instead of scheduling a separate worker to process RPC Replies,
promote the Receive completion handler to IB_POLL_WORKQUEUE, and
invoke rpcrdma_reply_handler directly.

Note that the poll workqueue is single-threaded. In order to keep
memory invalidation from serializing all RPC Replies, handle any
necessary invalidation tasks in a separate multi-threaded workqueue.

This provides a two-tier scheme, similar to OS I/O interrupt
handlers: A fast interrupt handler that schedules the slow handler
and re-enables the interrupt, and a slower handler that is invoked
for any needed heavy lifting.

Benefits include:
- One less context switch for RPCs that don't register memory
- Receive completion handling is moved out of soft IRQ context to
  make room for other users of soft IRQ
- The same CPU core now DMA syncs and XDR decodes the Receive buffer

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

3 files changed, 36 insertions, 23 deletions

diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
index 418bcc6b3e1d..430f8b5a8c43 100644
--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -1265,16 +1265,36 @@ out_badheader:
         goto out;
 }
 
+/* Reply handling runs in the poll worker thread. Anything that
+ * might wait is deferred to a separate workqueue.
+ */
+void rpcrdma_deferred_completion(struct work_struct *work)
+{
+        struct rpcrdma_rep *rep =
+                        container_of(work, struct rpcrdma_rep, rr_work);
+        struct rpcrdma_req *req = rpcr_to_rdmar(rep->rr_rqst);
+        struct rpcrdma_xprt *r_xprt = rep->rr_rxprt;
+
+        /* Invalidate and unmap the data payloads before waking
+         * the waiting application. This guarantees the memory
+         * regions are properly fenced from the server before the
+         * application accesses the data. It also ensures proper
+         * send flow control: waking the next RPC waits until this
+         * RPC has relinquished all its Send Queue entries.
+         */
+        rpcrdma_mark_remote_invalidation(&req->rl_registered, rep);
+        r_xprt->rx_ia.ri_ops->ro_unmap_sync(r_xprt, &req->rl_registered);
+
+        rpcrdma_complete_rqst(rep);
+}
+
 /* Process received RPC/RDMA messages.
  *
  * Errors must result in the RPC task either being awakened, or
  * allowed to timeout, to discover the errors at that time.
  */
-void
-rpcrdma_reply_handler(struct work_struct *work)
+void rpcrdma_reply_handler(struct rpcrdma_rep *rep)
 {
-        struct rpcrdma_rep *rep =
-                        container_of(work, struct rpcrdma_rep, rr_work);
         struct rpcrdma_xprt *r_xprt = rep->rr_rxprt;
         struct rpc_xprt *xprt = &r_xprt->rx_xprt;
         struct rpcrdma_req *req;
@@ -1320,20 +1340,10 @@ rpcrdma_reply_handler(struct work_struct *work)
         dprintk("RPC:       %s: reply %p completes request %p (xid 0x%08x)\n",
                 __func__, rep, req, be32_to_cpu(rep->rr_xid));
 
-        /* Invalidate and unmap the data payloads before waking the
-         * waiting application. This guarantees the memory regions
-         * are properly fenced from the server before the application
-         * accesses the data. It also ensures proper send flow control:
-         * waking the next RPC waits until this RPC has relinquished
-         * all its Send Queue entries.
-         */
-        if (!list_empty(&req->rl_registered)) {
-                rpcrdma_mark_remote_invalidation(&req->rl_registered, rep);
-                r_xprt->rx_ia.ri_ops->ro_unmap_sync(r_xprt,
-                                                    &req->rl_registered);
-        }
-
-        rpcrdma_complete_rqst(rep);
+        if (list_empty(&req->rl_registered))
+                rpcrdma_complete_rqst(rep);
+        else
+                queue_work(rpcrdma_receive_wq, &rep->rr_work);
         return;
 
 out_badstatus:
diff --git a/net/sunrpc/xprtrdma/verbs.c b/net/sunrpc/xprtrdma/verbs.c
index 11a1fbf7e59e..d45695408df3 100644
--- a/net/sunrpc/xprtrdma/verbs.c
+++ b/net/sunrpc/xprtrdma/verbs.c
@@ -73,7 +73,7 @@ static void rpcrdma_create_mrs(struct rpcrdma_xprt *r_xprt);
 static void rpcrdma_destroy_mrs(struct rpcrdma_buffer *buf);
 static void rpcrdma_dma_unmap_regbuf(struct rpcrdma_regbuf *rb);
 
-static struct workqueue_struct *rpcrdma_receive_wq __read_mostly;
+struct workqueue_struct *rpcrdma_receive_wq __read_mostly;
 
 int
 rpcrdma_alloc_wq(void)
@@ -185,7 +185,7 @@ rpcrdma_wc_receive(struct ib_cq *cq, struct ib_wc *wc)
                 rpcrdma_update_granted_credits(rep);
 
 out_schedule:
-        queue_work(rpcrdma_receive_wq, &rep->rr_work);
+        rpcrdma_reply_handler(rep);
         return;
 
 out_fail:
@@ -583,7 +583,7 @@ rpcrdma_ep_create(struct rpcrdma_ep *ep, struct rpcrdma_ia *ia,
 
         recvcq = ib_alloc_cq(ia->ri_device, NULL,
                              ep->rep_attr.cap.max_recv_wr + 1,
-                             0, IB_POLL_SOFTIRQ);
+                             0, IB_POLL_WORKQUEUE);
         if (IS_ERR(recvcq)) {
                 rc = PTR_ERR(recvcq);
                 dprintk("RPC:       %s: failed to create recv CQ: %i\n",
@@ -974,7 +974,7 @@ rpcrdma_create_rep(struct rpcrdma_xprt *r_xprt)
 
         rep->rr_cqe.done = rpcrdma_wc_receive;
         rep->rr_rxprt = r_xprt;
-        INIT_WORK(&rep->rr_work, rpcrdma_reply_handler);
+        INIT_WORK(&rep->rr_work, rpcrdma_deferred_completion);
         rep->rr_recv_wr.next = NULL;
         rep->rr_recv_wr.wr_cqe = &rep->rr_cqe;
         rep->rr_recv_wr.sg_list = &rep->rr_rdmabuf->rg_iov;
diff --git a/net/sunrpc/xprtrdma/xprt_rdma.h b/net/sunrpc/xprtrdma/xprt_rdma.h
index d68a1351d95e..a85bcd19b37a 100644
--- a/net/sunrpc/xprtrdma/xprt_rdma.h
+++ b/net/sunrpc/xprtrdma/xprt_rdma.h
@@ -533,6 +533,8 @@ void rpcrdma_ia_close(struct rpcrdma_ia *);
 bool frwr_is_supported(struct rpcrdma_ia *);
 bool fmr_is_supported(struct rpcrdma_ia *);
 
+extern struct workqueue_struct *rpcrdma_receive_wq;
+
 /*
  * Endpoint calls - xprtrdma/verbs.c
  */
@@ -617,7 +619,8 @@ void rpcrdma_unmap_sges(struct rpcrdma_ia *, struct rpcrdma_req *);
 int rpcrdma_marshal_req(struct rpcrdma_xprt *r_xprt, struct rpc_rqst *rqst);
 void rpcrdma_set_max_header_sizes(struct rpcrdma_xprt *);
 void rpcrdma_complete_rqst(struct rpcrdma_rep *rep);
-void rpcrdma_reply_handler(struct work_struct *work);
+void rpcrdma_reply_handler(struct rpcrdma_rep *rep);
+void rpcrdma_deferred_completion(struct work_struct *work);
 
 static inline void rpcrdma_set_xdrlen(struct xdr_buf *xdr, size_t len)
 {