svcrdma: Single-stage RDMA Read

Currently the generic RPC server layer calls svc_rdma_recvfrom()
twice to retrieve an RPC message that uses Read chunks. I'm not
exactly sure why this design was chosen originally.

Instead, let's wait for the Read chunk completion inline in the
first call to svc_rdma_recvfrom().

The goal is to eliminate some page allocator churn.
rdma_read_complete() replaces pages in the second svc_rqst by
calling put_page() repeatedly while the upper layer waits for the
request to be constructed, which adds unnecessary NFS WRITE round-
trip latency.

Signed-off-by: Chuck Lever <[email protected]>
Reviewed-by: Tom Talpey <[email protected]>

Author: chucklever

net/sunrpc/xprtrdma/svc_rdma_recvfrom.c

@@ -853,6 +853,9 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
 	spin_unlock(&rdma_xprt->sc_rq_dto_lock);
 	percpu_counter_inc(&svcrdma_stat_recv);
 
+	/* Unblock the transport for the next receive */
+	svc_xprt_received(xprt);
+
 	ib_dma_sync_single_for_cpu(rdma_xprt->sc_pd->device,
 				   ctxt->rc_recv_sge.addr, ctxt->rc_byte_len,
 				   DMA_FROM_DEVICE);
@@ -884,33 +887,28 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
 	rqstp->rq_xprt_ctxt = ctxt;
 	rqstp->rq_prot = IPPROTO_MAX;
 	svc_xprt_copy_addrs(rqstp, xprt);
-	svc_xprt_received(xprt);
 	return rqstp->rq_arg.len;
 
 out_readlist:
 	ret = svc_rdma_process_read_list(rdma_xprt, rqstp, ctxt);
 	if (ret < 0)
 		goto out_readfail;
-	svc_xprt_received(xprt);
-	return 0;
+	goto complete;
 
 out_err:
 	svc_rdma_send_error(rdma_xprt, ctxt, ret);
 	svc_rdma_recv_ctxt_put(rdma_xprt, ctxt);
-	svc_xprt_received(xprt);
 	return 0;
 
 out_readfail:
 	if (ret == -EINVAL)
 		svc_rdma_send_error(rdma_xprt, ctxt, ret);
 	svc_rdma_recv_ctxt_put(rdma_xprt, ctxt);
-	svc_xprt_received(xprt);
 	return ret;
 
 out_backchannel:
 	svc_rdma_handle_bc_reply(rqstp, ctxt);
 out_drop:
 	svc_rdma_recv_ctxt_put(rdma_xprt, ctxt);
-	svc_xprt_received(xprt);
 	return 0;
 }

net/sunrpc/xprtrdma/svc_rdma_rw.c

@@ -150,6 +150,8 @@ struct svc_rdma_chunk_ctxt {
 	struct svcxprt_rdma	*cc_rdma;
 	struct list_head	cc_rwctxts;
 	int			cc_sqecount;
+	enum ib_wc_status	cc_status;
+	struct completion	cc_done;
 };
 
 static void svc_rdma_cc_cid_init(struct svcxprt_rdma *rdma,
@@ -299,29 +301,15 @@ static void svc_rdma_wc_read_done(struct ib_cq *cq, struct ib_wc *wc)
 	struct svc_rdma_chunk_ctxt *cc =
 			container_of(cqe, struct svc_rdma_chunk_ctxt, cc_cqe);
 	struct svcxprt_rdma *rdma = cc->cc_rdma;
-	struct svc_rdma_read_info *info =
-			container_of(cc, struct svc_rdma_read_info, ri_cc);
 
 	trace_svcrdma_wc_read(wc, &cc->cc_cid);
 
 	atomic_add(cc->cc_sqecount, &rdma->sc_sq_avail);
 	wake_up(&rdma->sc_send_wait);
 
-	if (unlikely(wc->status != IB_WC_SUCCESS)) {
-		set_bit(XPT_CLOSE, &rdma->sc_xprt.xpt_flags);
-		svc_rdma_recv_ctxt_put(rdma, info->ri_readctxt);
-	} else {
-		spin_lock(&rdma->sc_rq_dto_lock);
-		list_add_tail(&info->ri_readctxt->rc_list,
-			      &rdma->sc_read_complete_q);
-		/* Note the unlock pairs with the smp_rmb in svc_xprt_ready: */
-		set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags);
-		spin_unlock(&rdma->sc_rq_dto_lock);
-
-		svc_xprt_enqueue(&rdma->sc_xprt);
-	}
-
-	svc_rdma_read_info_free(info);
+	cc->cc_status = wc->status;
+	complete(&cc->cc_done);
+	return;
 }
 
 /* This function sleeps when the transport's Send Queue is congested.
@@ -676,8 +664,8 @@ static int svc_rdma_build_read_segment(struct svc_rdma_read_info *info,
 	struct svc_rdma_recv_ctxt *head = info->ri_readctxt;
 	struct svc_rdma_chunk_ctxt *cc = &info->ri_cc;
 	struct svc_rqst *rqstp = info->ri_rqst;
-	struct svc_rdma_rw_ctxt *ctxt;
 	unsigned int sge_no, seg_len, len;
+	struct svc_rdma_rw_ctxt *ctxt;
 	struct scatterlist *sg;
 	int ret;
 
@@ -693,8 +681,6 @@ static int svc_rdma_build_read_segment(struct svc_rdma_read_info *info,
 		seg_len = min_t(unsigned int, len,
 				PAGE_SIZE - info->ri_pageoff);
 
-		head->rc_arg.pages[info->ri_pageno] =
-			rqstp->rq_pages[info->ri_pageno];
 		if (!info->ri_pageoff)
 			head->rc_page_count++;
 
@@ -788,12 +774,10 @@ static int svc_rdma_copy_inline_range(struct svc_rdma_read_info *info,
 		page_len = min_t(unsigned int, remaining,
 				 PAGE_SIZE - info->ri_pageoff);
 
-		head->rc_arg.pages[info->ri_pageno] =
-			rqstp->rq_pages[info->ri_pageno];
 		if (!info->ri_pageoff)
 			head->rc_page_count++;
 
-		dst = page_address(head->rc_arg.pages[info->ri_pageno]);
+		dst = page_address(rqstp->rq_pages[info->ri_pageno]);
 		memcpy(dst + info->ri_pageno, src + offset, page_len);
 
 		info->ri_totalbytes += page_len;
@@ -813,7 +797,7 @@ static int svc_rdma_copy_inline_range(struct svc_rdma_read_info *info,
  * svc_rdma_read_multiple_chunks - Construct RDMA Reads to pull data item Read chunks
  * @info: context for RDMA Reads
  *
- * The chunk data lands in head->rc_arg as a series of contiguous pages,
+ * The chunk data lands in rqstp->rq_arg as a series of contiguous pages,
  * like an incoming TCP call.
  *
  * Return values:
@@ -827,8 +811,8 @@ static noinline int svc_rdma_read_multiple_chunks(struct svc_rdma_read_info *inf
 {
 	struct svc_rdma_recv_ctxt *head = info->ri_readctxt;
 	const struct svc_rdma_pcl *pcl = &head->rc_read_pcl;
+	struct xdr_buf *buf = &info->ri_rqst->rq_arg;
 	struct svc_rdma_chunk *chunk, *next;
-	struct xdr_buf *buf = &head->rc_arg;
 	unsigned int start, length;
 	int ret;
 
@@ -864,9 +848,9 @@ static noinline int svc_rdma_read_multiple_chunks(struct svc_rdma_read_info *inf
 	buf->len += info->ri_totalbytes;
 	buf->buflen += info->ri_totalbytes;
 
-	head->rc_hdr_count = 1;
-	buf->head[0].iov_base = page_address(head->rc_pages[0]);
+	buf->head[0].iov_base = page_address(info->ri_rqst->rq_pages[0]);
 	buf->head[0].iov_len = min_t(size_t, PAGE_SIZE, info->ri_totalbytes);
+	buf->pages = &info->ri_rqst->rq_pages[1];
 	buf->page_len = info->ri_totalbytes - buf->head[0].iov_len;
 	return 0;
 }
@@ -875,9 +859,9 @@ static noinline int svc_rdma_read_multiple_chunks(struct svc_rdma_read_info *inf
  * svc_rdma_read_data_item - Construct RDMA Reads to pull data item Read chunks
  * @info: context for RDMA Reads
  *
- * The chunk data lands in the page list of head->rc_arg.pages.
+ * The chunk data lands in the page list of rqstp->rq_arg.pages.
  *
- * Currently NFSD does not look at the head->rc_arg.tail[0] iovec.
+ * Currently NFSD does not look at the rqstp->rq_arg.tail[0] kvec.
  * Therefore, XDR round-up of the Read chunk and trailing
  * inline content must both be added at the end of the pagelist.
  *
@@ -891,7 +875,7 @@ static noinline int svc_rdma_read_multiple_chunks(struct svc_rdma_read_info *inf
 static int svc_rdma_read_data_item(struct svc_rdma_read_info *info)
 {
 	struct svc_rdma_recv_ctxt *head = info->ri_readctxt;
-	struct xdr_buf *buf = &head->rc_arg;
+	struct xdr_buf *buf = &info->ri_rqst->rq_arg;
 	struct svc_rdma_chunk *chunk;
 	unsigned int length;
 	int ret;
@@ -901,8 +885,6 @@ static int svc_rdma_read_data_item(struct svc_rdma_read_info *info)
 	if (ret < 0)
 		goto out;
 
-	head->rc_hdr_count = 0;
-
 	/* Split the Receive buffer between the head and tail
 	 * buffers at Read chunk's position. XDR roundup of the
 	 * chunk is not included in either the pagelist or in
@@ -921,7 +903,8 @@ static int svc_rdma_read_data_item(struct svc_rdma_read_info *info)
 	 * Currently these chunks always start at page offset 0,
 	 * thus the rounded-up length never crosses a page boundary.
 	 */
-	length = XDR_QUADLEN(info->ri_totalbytes) << 2;
+	buf->pages = &info->ri_rqst->rq_pages[0];
+	length = xdr_align_size(chunk->ch_length);
 	buf->page_len = length;
 	buf->len += length;
 	buf->buflen += length;
@@ -1033,8 +1016,7 @@ static int svc_rdma_read_call_chunk(struct svc_rdma_read_info *info)
  * @info: context for RDMA Reads
  *
  * The start of the data lands in the first page just after the
- * Transport header, and the rest lands in the page list of
- * head->rc_arg.pages.
+ * Transport header, and the rest lands in rqstp->rq_arg.pages.
  *
  * Assumptions:
  *	- A PZRC is never sent in an RDMA_MSG message, though it's
@@ -1049,8 +1031,7 @@ static int svc_rdma_read_call_chunk(struct svc_rdma_read_info *info)
  */
 static noinline int svc_rdma_read_special(struct svc_rdma_read_info *info)
 {
-	struct svc_rdma_recv_ctxt *head = info->ri_readctxt;
-	struct xdr_buf *buf = &head->rc_arg;
+	struct xdr_buf *buf = &info->ri_rqst->rq_arg;
 	int ret;
 
 	ret = svc_rdma_read_call_chunk(info);
@@ -1060,35 +1041,15 @@ static noinline int svc_rdma_read_special(struct svc_rdma_read_info *info)
 	buf->len += info->ri_totalbytes;
 	buf->buflen += info->ri_totalbytes;
 
-	head->rc_hdr_count = 1;
-	buf->head[0].iov_base = page_address(head->rc_pages[0]);
+	buf->head[0].iov_base = page_address(info->ri_rqst->rq_pages[0]);
 	buf->head[0].iov_len = min_t(size_t, PAGE_SIZE, info->ri_totalbytes);
+	buf->pages = &info->ri_rqst->rq_pages[1];
 	buf->page_len = info->ri_totalbytes - buf->head[0].iov_len;
 
 out:
 	return ret;
 }
 
-/* Pages under I/O have been copied to head->rc_pages. Ensure they
- * are not released by svc_xprt_release() until the I/O is complete.
- *
- * This has to be done after all Read WRs are constructed to properly
- * handle a page that is part of I/O on behalf of two different RDMA
- * segments.
- *
- * Do this only if I/O has been posted. Otherwise, we do indeed want
- * svc_xprt_release() to clean things up properly.
- */
-static void svc_rdma_save_io_pages(struct svc_rqst *rqstp,
-				   const unsigned int start,
-				   const unsigned int num_pages)
-{
-	unsigned int i;
-
-	for (i = start; i < num_pages + start; i++)
-		rqstp->rq_pages[i] = NULL;
-}
-
 /**
  * svc_rdma_process_read_list - Pull list of Read chunks from the client
  * @rdma: controlling RDMA transport
@@ -1153,11 +1114,22 @@ int svc_rdma_process_read_list(struct svcxprt_rdma *rdma,
 		goto out_err;
 
 	trace_svcrdma_post_read_chunk(&cc->cc_cid, cc->cc_sqecount);
+	init_completion(&cc->cc_done);
 	ret = svc_rdma_post_chunk_ctxt(cc);
 	if (ret < 0)
 		goto out_err;
-	svc_rdma_save_io_pages(rqstp, 0, head->rc_page_count);
-	return 1;
+
+	ret = 1;
+	wait_for_completion(&cc->cc_done);
+	if (cc->cc_status != IB_WC_SUCCESS)
+		ret = -EIO;
+
+	/* rq_respages starts after the last arg page */
+	rqstp->rq_respages = &rqstp->rq_pages[head->rc_page_count];
+	rqstp->rq_next_page = rqstp->rq_respages + 1;
+
+	/* Ensure svc_rdma_recv_ctxt_put() does not try to release pages */
+	head->rc_page_count = 0;
 
 out_err:
 	svc_rdma_read_info_free(info);