xprtrdma: Add data structure to manage RDMA Send arguments

Problem statement: Recently Sagi Grimberg <sagi@grimberg.me> observed that kernel RDMA- enabled storage initiators don't handle delayed Send completion correctly. If Send completion is delayed beyond the end of a ULP transaction, the ULP may release resources that are still being used by the HCA to complete a long-running Send operation. This is a common design trait amongst our initiators. Most Send operations are faster than the ULP transaction they are part of. Waiting for a completion for these is typically unnecessary. Infrequently, a network partition or some other problem crops up where an ordering problem can occur. In NFS parlance, the RPC Reply arrives and completes the RPC, but the HCA is still retrying the Send WR that conveyed the RPC Call. In this case, the HCA can try to use memory that has been invalidated or DMA unmapped, and the connection is lost. If that memory has been re-used for something else (possibly not related to NFS), and the Send retransmission exposes that data on the wire. Thus we cannot assume that it is safe to release Send-related resources just because a ULP reply has arrived. After some analysis, we have determined that the completion housekeeping will not be difficult for xprtrdma: - Inline Send buffers are registered via the local DMA key, and are already left DMA mapped for the lifetime of a transport connection, thus no additional handling is necessary for those - Gathered Sends involving page cache pages _will_ need to DMA unmap those pages after the Send completes. But like inline send buffers, they are registered via the local DMA key, and thus will not need to be invalidated In addition, RPC completion will need to wait for Send completion in the latter case. However, nearly always, the Send that conveys the RPC Call will have completed long before the RPC Reply arrives, and thus no additional latency will be accrued. Design notes: In this patch, the rpcrdma_sendctx object is introduced, and a lock-free circular queue is added to manage a set of them per transport. The RPC client's send path already prevents sending more than one RPC Call at the same time. This allows us to treat the consumer side of the queue (rpcrdma_sendctx_get_locked) as if there is a single consumer thread. The producer side of the queue (rpcrdma_sendctx_put_locked) is invoked only from the Send completion handler, which is a single thread of execution (soft IRQ). The only care that needs to be taken is with the tail index, which is shared between the producer and consumer. Only the producer updates the tail index. The consumer compares the head with the tail to ensure that the a sendctx that is in use is never handed out again (or, expressed more conventionally, the queue is empty). When the sendctx queue empties completely, there are enough Sends outstanding that posting more Send operations can result in a Send Queue overflow. In this case, the ULP is told to wait and try again. This introduces strong Send Queue accounting to xprtrdma. As a final touch, Jason Gunthorpe <jgunthorpe@obsidianresearch.com> suggested a mechanism that does not require signaling every Send. We signal once every N Sends, and perform SGE unmapping of N Send operations during that one completion. Reported-by: Sagi Grimberg <sagi@grimberg.me> Suggested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com> Signed-off-by: Chuck Lever <chuck.lever@oracle.com> Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
2017-10-20 10:48:12 -04:00
parent a062a2a3ef
commit ae72950abf
4 changed files with 247 additions and 32 deletions
--- a/net/sunrpc/xprtrdma/verbs.c
+++ b/net/sunrpc/xprtrdma/verbs.c
@@ -52,6 +52,8 @@
 #include <linux/prefetch.h>
 #include <linux/sunrpc/addr.h>
 #include <linux/sunrpc/svc_rdma.h>
+
+#include <asm-generic/barrier.h>
 #include <asm/bitops.h>

 #include <rdma/ib_cm.h>
@@ -126,11 +128,17 @@ rpcrdma_qp_async_error_upcall(struct ib_event *event, void *context)
 static void
 rpcrdma_wc_send(struct ib_cq *cq, struct ib_wc *wc)
 {
+	struct ib_cqe *cqe = wc->wr_cqe;
+	struct rpcrdma_sendctx *sc =
+		container_of(cqe, struct rpcrdma_sendctx, sc_cqe);
+
 	/* WARNING: Only wr_cqe and status are reliable at this point */
 	if (wc->status != IB_WC_SUCCESS && wc->status != IB_WC_WR_FLUSH_ERR)
 		pr_err("rpcrdma: Send: %s (%u/0x%x)\n",
 		       ib_wc_status_msg(wc->status),
 		       wc->status, wc->vendor_err);
+
+	rpcrdma_sendctx_put_locked(sc);
 }

 /**
@@ -542,6 +550,9 @@ rpcrdma_ep_create(struct rpcrdma_ep *ep, struct rpcrdma_ia *ia,
 		ep->rep_attr.cap.max_recv_sge);

 	/* set trigger for requesting send completion */
+	ep->rep_send_batch = min_t(unsigned int, RPCRDMA_MAX_SEND_BATCH,
+				   cdata->max_requests >> 2);
+	ep->rep_send_count = ep->rep_send_batch;
 	ep->rep_cqinit = ep->rep_attr.cap.max_send_wr/2 - 1;
 	if (ep->rep_cqinit <= 2)
 		ep->rep_cqinit = 0;	/* always signal? */
@@ -824,6 +835,168 @@ rpcrdma_ep_disconnect(struct rpcrdma_ep *ep, struct rpcrdma_ia *ia)
 	ib_drain_qp(ia->ri_id->qp);
 }

+/* Fixed-size circular FIFO queue. This implementation is wait-free and
+ * lock-free.
+ *
+ * Consumer is the code path that posts Sends. This path dequeues a
+ * sendctx for use by a Send operation. Multiple consumer threads
+ * are serialized by the RPC transport lock, which allows only one
+ * ->send_request call at a time.
+ *
+ * Producer is the code path that handles Send completions. This path
+ * enqueues a sendctx that has been completed. Multiple producer
+ * threads are serialized by the ib_poll_cq() function.
+ */
+
+/* rpcrdma_sendctxs_destroy() assumes caller has already quiesced
+ * queue activity, and ib_drain_qp has flushed all remaining Send
+ * requests.
+ */
+static void rpcrdma_sendctxs_destroy(struct rpcrdma_buffer *buf)
+{
+	unsigned long i;
+
+	for (i = 0; i <= buf->rb_sc_last; i++)
+		kfree(buf->rb_sc_ctxs[i]);
+	kfree(buf->rb_sc_ctxs);
+}
+
+static struct rpcrdma_sendctx *rpcrdma_sendctx_create(struct rpcrdma_ia *ia)
+{
+	struct rpcrdma_sendctx *sc;
+
+	sc = kzalloc(sizeof(*sc) +
+		     ia->ri_max_send_sges * sizeof(struct ib_sge),
+		     GFP_KERNEL);
+	if (!sc)
+		return NULL;
+
+	sc->sc_wr.wr_cqe = &sc->sc_cqe;
+	sc->sc_wr.sg_list = sc->sc_sges;
+	sc->sc_wr.opcode = IB_WR_SEND;
+	sc->sc_cqe.done = rpcrdma_wc_send;
+	return sc;
+}
+
+static int rpcrdma_sendctxs_create(struct rpcrdma_xprt *r_xprt)
+{
+	struct rpcrdma_buffer *buf = &r_xprt->rx_buf;
+	struct rpcrdma_sendctx *sc;
+	unsigned long i;
+
+	/* Maximum number of concurrent outstanding Send WRs. Capping
+	 * the circular queue size stops Send Queue overflow by causing
+	 * the ->send_request call to fail temporarily before too many
+	 * Sends are posted.
+	 */
+	i = buf->rb_max_requests + RPCRDMA_MAX_BC_REQUESTS;
+	dprintk("RPC:       %s: allocating %lu send_ctxs\n", __func__, i);
+	buf->rb_sc_ctxs = kcalloc(i, sizeof(sc), GFP_KERNEL);
+	if (!buf->rb_sc_ctxs)
+		return -ENOMEM;
+
+	buf->rb_sc_last = i - 1;
+	for (i = 0; i <= buf->rb_sc_last; i++) {
+		sc = rpcrdma_sendctx_create(&r_xprt->rx_ia);
+		if (!sc)
+			goto out_destroy;
+
+		sc->sc_xprt = r_xprt;
+		buf->rb_sc_ctxs[i] = sc;
+	}
+
+	return 0;
+
+out_destroy:
+	rpcrdma_sendctxs_destroy(buf);
+	return -ENOMEM;
+}
+
+/* The sendctx queue is not guaranteed to have a size that is a
+ * power of two, thus the helpers in circ_buf.h cannot be used.
+ * The other option is to use modulus (%), which can be expensive.
+ */
+static unsigned long rpcrdma_sendctx_next(struct rpcrdma_buffer *buf,
+					  unsigned long item)
+{
+	return likely(item < buf->rb_sc_last) ? item + 1 : 0;
+}
+
+/**
+ * rpcrdma_sendctx_get_locked - Acquire a send context
+ * @buf: transport buffers from which to acquire an unused context
+ *
+ * Returns pointer to a free send completion context; or NULL if
+ * the queue is empty.
+ *
+ * Usage: Called to acquire an SGE array before preparing a Send WR.
+ *
+ * The caller serializes calls to this function (per rpcrdma_buffer),
+ * and provides an effective memory barrier that flushes the new value
+ * of rb_sc_head.
+ */
+struct rpcrdma_sendctx *rpcrdma_sendctx_get_locked(struct rpcrdma_buffer *buf)
+{
+	struct rpcrdma_xprt *r_xprt;
+	struct rpcrdma_sendctx *sc;
+	unsigned long next_head;
+
+	next_head = rpcrdma_sendctx_next(buf, buf->rb_sc_head);
+
+	if (next_head == READ_ONCE(buf->rb_sc_tail))
+		goto out_emptyq;
+
+	/* ORDER: item must be accessed _before_ head is updated */
+	sc = buf->rb_sc_ctxs[next_head];
+
+	/* Releasing the lock in the caller acts as a memory
+	 * barrier that flushes rb_sc_head.
+	 */
+	buf->rb_sc_head = next_head;
+
+	return sc;
+
+out_emptyq:
+	/* The queue is "empty" if there have not been enough Send
+	 * 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__);
+	r_xprt = container_of(buf, struct rpcrdma_xprt, rx_buf);
+	r_xprt->rx_stats.empty_sendctx_q++;
+	return NULL;
+}
+
+/**
+ * rpcrdma_sendctx_put_locked - Release a send context
+ * @sc: send context to release
+ *
+ * Usage: Called from Send completion to return a sendctxt
+ * to the queue.
+ *
+ * The caller serializes calls to this function (per rpcrdma_buffer).
+ */
+void rpcrdma_sendctx_put_locked(struct rpcrdma_sendctx *sc)
+{
+	struct rpcrdma_buffer *buf = &sc->sc_xprt->rx_buf;
+	unsigned long next_tail;
+
+	/* Unmap SGEs of previously completed by unsignaled
+	 * Sends by walking up the queue until @sc is found.
+	 */
+	next_tail = buf->rb_sc_tail;
+	do {
+		next_tail = rpcrdma_sendctx_next(buf, next_tail);
+
+		/* ORDER: item must be accessed _before_ tail is updated */
+		rpcrdma_unmap_sendctx(buf->rb_sc_ctxs[next_tail]);
+
+	} while (buf->rb_sc_ctxs[next_tail] != sc);
+
+	/* Paired with READ_ONCE */
+	smp_store_release(&buf->rb_sc_tail, next_tail);
+}
+
 static void
 rpcrdma_mr_recovery_worker(struct work_struct *work)
 {
@@ -919,13 +1092,8 @@ rpcrdma_create_req(struct rpcrdma_xprt *r_xprt)
 	spin_lock(&buffer->rb_reqslock);
 	list_add(&req->rl_all, &buffer->rb_allreqs);
 	spin_unlock(&buffer->rb_reqslock);
-	req->rl_cqe.done = rpcrdma_wc_send;
 	req->rl_buffer = &r_xprt->rx_buf;
 	INIT_LIST_HEAD(&req->rl_registered);
-	req->rl_send_wr.next = NULL;
-	req->rl_send_wr.wr_cqe = &req->rl_cqe;
-	req->rl_send_wr.sg_list = req->rl_send_sge;
-	req->rl_send_wr.opcode = IB_WR_SEND;
 	return req;
 }

@@ -1017,6 +1185,10 @@ rpcrdma_buffer_create(struct rpcrdma_xprt *r_xprt)
 		list_add(&rep->rr_list, &buf->rb_recv_bufs);
 	}

+	rc = rpcrdma_sendctxs_create(r_xprt);
+	if (rc)
+		goto out;
+
 	return 0;
 out:
 	rpcrdma_buffer_destroy(buf);
@@ -1093,6 +1265,8 @@ rpcrdma_buffer_destroy(struct rpcrdma_buffer *buf)
 	cancel_delayed_work_sync(&buf->rb_recovery_worker);
 	cancel_delayed_work_sync(&buf->rb_refresh_worker);

+	rpcrdma_sendctxs_destroy(buf);
+
 	while (!list_empty(&buf->rb_recv_bufs)) {
 		struct rpcrdma_rep *rep;

@@ -1208,7 +1382,6 @@ rpcrdma_buffer_put(struct rpcrdma_req *req)
 	struct rpcrdma_buffer *buffers = req->rl_buffer;
 	struct rpcrdma_rep *rep = req->rl_reply;

-	req->rl_send_wr.num_sge = 0;
 	req->rl_reply = NULL;

 	spin_lock(&buffers->rb_lock);
@@ -1340,7 +1513,7 @@ rpcrdma_ep_post(struct rpcrdma_ia *ia,
 		struct rpcrdma_ep *ep,
 		struct rpcrdma_req *req)
 {
-	struct ib_send_wr *send_wr = &req->rl_send_wr;
+	struct ib_send_wr *send_wr = &req->rl_sendctx->sc_wr;
 	struct ib_send_wr *send_wr_fail;
 	int rc;

@@ -1354,7 +1527,13 @@ rpcrdma_ep_post(struct rpcrdma_ia *ia,
 	dprintk("RPC:       %s: posting %d s/g entries\n",
 		__func__, send_wr->num_sge);

-	rpcrdma_set_signaled(ep, send_wr);
+	if (!ep->rep_send_count) {
+		send_wr->send_flags |= IB_SEND_SIGNALED;
+		ep->rep_send_count = ep->rep_send_batch;
+	} else {
+		send_wr->send_flags &= ~IB_SEND_SIGNALED;
+		--ep->rep_send_count;
+	}
 	rc = ib_post_send(ia->ri_id->qp, send_wr, &send_wr_fail);
 	if (rc)
 		goto out_postsend_err;