svcrdma: Convert rdma->sc_rw_ctxts to llist

Relieve contention on sc_rw_ctxt_lock by converting rdma->sc_rw_ctxts
to an llist.

The goal is to reduce the average overhead of Send completions,
because a transport's completion handlers are single-threaded on
one CPU core. This change reduces CPU utilization of each Send
completion by 2-3% on my server.

Signed-off-by: Chuck Lever <[e-mail]>
Reviewed-By: Tom Talpey <[e-mail]>

include/linux/sunrpc/svc_rdma.h

···
 	spinlock_t	     sc_send_lock;
 	struct llist_head    sc_send_ctxts;
 	spinlock_t	     sc_rw_ctxt_lock;
-	struct list_head     sc_rw_ctxts;
+	struct llist_head    sc_rw_ctxts;
 
 	u32		     sc_pending_recvs;
 	u32		     sc_recv_batch;

net/sunrpc/xprtrdma/svc_rdma_rw.c

···
  * controlling svcxprt_rdma is destroyed.
  */
 struct svc_rdma_rw_ctxt {
+	struct llist_node	rw_node;
 	struct list_head	rw_list;
 	struct rdma_rw_ctx	rw_ctx;
 	unsigned int		rw_nents;
···
 svc_rdma_get_rw_ctxt(struct svcxprt_rdma *rdma, unsigned int sges)
 {
 	struct svc_rdma_rw_ctxt *ctxt;
+	struct llist_node *node;
 
 	spin_lock(&rdma->sc_rw_ctxt_lock);
-
-	ctxt = svc_rdma_next_ctxt(&rdma->sc_rw_ctxts);
-	if (ctxt) {
-		list_del(&ctxt->rw_list);
-		spin_unlock(&rdma->sc_rw_ctxt_lock);
+	node = llist_del_first(&rdma->sc_rw_ctxts);
+	spin_unlock(&rdma->sc_rw_ctxt_lock);
+	if (node) {
+		ctxt = llist_entry(node, struct svc_rdma_rw_ctxt, rw_node);
 	} else {
-		spin_unlock(&rdma->sc_rw_ctxt_lock);
 		ctxt = kmalloc(struct_size(ctxt, rw_first_sgl, SG_CHUNK_SIZE),
 			       GFP_KERNEL);
 		if (!ctxt)
 			goto out_noctx;
+
 		INIT_LIST_HEAD(&ctxt->rw_list);
 	}
 
···
 	return NULL;
 }
 
+static void __svc_rdma_put_rw_ctxt(struct svcxprt_rdma *rdma,
+				   struct svc_rdma_rw_ctxt *ctxt,
+				   struct llist_head *list)
+{
+	sg_free_table_chained(&ctxt->rw_sg_table, SG_CHUNK_SIZE);
+	llist_add(&ctxt->rw_node, list);
+}
+
 static void svc_rdma_put_rw_ctxt(struct svcxprt_rdma *rdma,
 				 struct svc_rdma_rw_ctxt *ctxt)
 {
-	sg_free_table_chained(&ctxt->rw_sg_table, SG_CHUNK_SIZE);
-
-	spin_lock(&rdma->sc_rw_ctxt_lock);
-	list_add(&ctxt->rw_list, &rdma->sc_rw_ctxts);
-	spin_unlock(&rdma->sc_rw_ctxt_lock);
+	__svc_rdma_put_rw_ctxt(rdma, ctxt, &rdma->sc_rw_ctxts);
 }
 
 /**
···
 void svc_rdma_destroy_rw_ctxts(struct svcxprt_rdma *rdma)
 {
 	struct svc_rdma_rw_ctxt *ctxt;
+	struct llist_node *node;
 
-	while ((ctxt = svc_rdma_next_ctxt(&rdma->sc_rw_ctxts)) != NULL) {
-		list_del(&ctxt->rw_list);
+	while ((node = llist_del_first(&rdma->sc_rw_ctxts)) != NULL) {
+		ctxt = llist_entry(node, struct svc_rdma_rw_ctxt, rw_node);
 		kfree(ctxt);
 	}
 }
···
 	cc->cc_sqecount = 0;
 }
 
+/*
+ * The consumed rw_ctx's are cleaned and placed on a local llist so
+ * that only one atomic llist operation is needed to put them all
+ * back on the free list.
+ */
 static void svc_rdma_cc_release(struct svc_rdma_chunk_ctxt *cc,
 				enum dma_data_direction dir)
 {
 	struct svcxprt_rdma *rdma = cc->cc_rdma;
+	struct llist_node *first, *last;
 	struct svc_rdma_rw_ctxt *ctxt;
+	LLIST_HEAD(free);
 
+	first = last = NULL;
 	while ((ctxt = svc_rdma_next_ctxt(&cc->cc_rwctxts)) != NULL) {
 		list_del(&ctxt->rw_list);
 
 		rdma_rw_ctx_destroy(&ctxt->rw_ctx, rdma->sc_qp,
 				    rdma->sc_port_num, ctxt->rw_sg_table.sgl,
 				    ctxt->rw_nents, dir);
-		svc_rdma_put_rw_ctxt(rdma, ctxt);
+		__svc_rdma_put_rw_ctxt(rdma, ctxt, &free);
+
+		ctxt->rw_node.next = first;
+		first = &ctxt->rw_node;
+		if (!last)
+			last = first;
 	}
+	if (first)
+		llist_add_batch(first, last, &rdma->sc_rw_ctxts);
 }
 
 /* State for sending a Write or Reply chunk.

net/sunrpc/xprtrdma/svc_rdma_transport.c

···
 	INIT_LIST_HEAD(&cma_xprt->sc_rq_dto_q);
 	init_llist_head(&cma_xprt->sc_send_ctxts);
 	init_llist_head(&cma_xprt->sc_recv_ctxts);
-	INIT_LIST_HEAD(&cma_xprt->sc_rw_ctxts);
+	init_llist_head(&cma_xprt->sc_rw_ctxts);
 	init_waitqueue_head(&cma_xprt->sc_send_wait);
 
 	spin_lock_init(&cma_xprt->sc_lock);