Problem statement:
Recently Sagi Grimberg <sagi@xxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx>
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@xxxxxxxxxxx>
Suggested-by: Jason Gunthorpe <jgunthorpe@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---
net/sunrpc/xprtrdma/rpc_rdma.c | 22 +++++
net/sunrpc/xprtrdma/transport.c | 5 +
net/sunrpc/xprtrdma/verbs.c | 169 +++++++++++++++++++++++++++++++++++++++
net/sunrpc/xprtrdma/xprt_rdma.h | 41 +++++++++
4 files changed, 235 insertions(+), 2 deletions(-)
diff --git a/net/sunrpc/xprtrdma/rpc_rdma.c b/net/sunrpc/xprtrdma/rpc_rdma.c
index 0db7ba0..62ec1f7 100644
--- a/net/sunrpc/xprtrdma/rpc_rdma.c
+++ b/net/sunrpc/xprtrdma/rpc_rdma.c
@@ -697,6 +697,28 @@ rpcrdma_unmap_sges(struct rpcrdma_ia *ia, struct rpcrdma_req *req)
}
/**
+ * __rpcrdma_unmap_send_sges - DMA-unmap Send buffers
+ * @sc: send_ctx containing SGEs to unmap
+ *
+ * The first two SGEs are always left mapped. They contain the transport
+ * header and the inline buffer.
+ */
+void
+__rpcrdma_unmap_send_sges(struct rpcrdma_sendctx *sc)
+{
+ struct ib_sge *sge;
+ unsigned int count;
+
+ dprintk("RPC: %s: unmapping %u sges for sc=%p\n",
+ __func__, sc->sc_unmap_count, sc);
+
+ sge = &sc->sc_sges[2];
+ for (count = sc->sc_unmap_count; count; ++sge, --count)
+ ib_dma_unmap_page(sc->sc_device, sge->addr, sge->length,
+ DMA_TO_DEVICE);
+}
+
+/**
* rpcrdma_marshal_req - Marshal and send one RPC request
* @r_xprt: controlling transport
* @rqst: RPC request to be marshaled
diff --git a/net/sunrpc/xprtrdma/transport.c b/net/sunrpc/xprtrdma/transport.c
index b680591..18cb8b4 100644
--- a/net/sunrpc/xprtrdma/transport.c
+++ b/net/sunrpc/xprtrdma/transport.c
@@ -790,11 +790,12 @@ void xprt_rdma_print_stats(struct rpc_xprt *xprt, struct seq_file *seq)
r_xprt->rx_stats.failed_marshal_count,
r_xprt->rx_stats.bad_reply_count,
r_xprt->rx_stats.nomsg_call_count);
- seq_printf(seq, "%lu %lu %lu %lu\n",
+ seq_printf(seq, "%lu %lu %lu %lu %lu\n",
r_xprt->rx_stats.mrs_recovered,
r_xprt->rx_stats.mrs_orphaned,
r_xprt->rx_stats.mrs_allocated,
- r_xprt->rx_stats.local_inv_needed);
+ r_xprt->rx_stats.local_inv_needed,
+ r_xprt->rx_stats.empty_sendctx_q);
}
static int
diff --git a/net/sunrpc/xprtrdma/verbs.c b/net/sunrpc/xprtrdma/verbs.c
index c78fb27..219f2d8 100644
--- 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>
@@ -564,6 +566,9 @@ 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? */
@@ -846,6 +851,164 @@ 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 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)
+{
+ kfree(buf->rb_sc_ctxs);
+}
+
+static struct rpcrdma_sendctx *rpcrdma_sendctx_create(unsigned int max_sges)
+{
+ struct rpcrdma_sendctx *sc;
+
+ sc = kzalloc(sizeof(*sc) + max_sges * sizeof(struct ib_sge),
+ GFP_KERNEL);
+ if (!sc)
+ return NULL;
+
+ sc->sc_wr.next = 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;
+ unsigned long i;
+
+ /* Maximum number of concurrent outstanding Send WRs */
+ i = buf->rb_max_requests + RPCRDMA_MAX_BC_REQUESTS;
+ dprintk("RPC: %s: allocating %lu send_ctxs (batch size %u)\n",
+ __func__, i, r_xprt->rx_ep.rep_send_batch);
+ buf->rb_sc_ctxs = kcalloc(i, sizeof(struct rpcrdma_sendctx *),
+ GFP_KERNEL);
+ if (!buf->rb_sc_ctxs)
+ return -ENOMEM;
+
+ buf->rb_sc_last = i - 1;
+ for (i = 0; i <= buf->rb_sc_last; i++) {
+ struct rpcrdma_sendctx *sc;
+
+ sc = rpcrdma_sendctx_create(r_xprt->rx_ia.ri_max_send_sges);
+ if (!sc)
+ goto out_destroy;
+
+ sc->sc_buffers = buf;
+ 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 before Send WR is posted to acquire an
+ * SGE array.
+ *
+ * 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 when Send completes to return a
+ * completed send context 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_buffers;
+ 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_send_sges(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)
{
@@ -1041,6 +1204,10 @@ 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);
@@ -1117,6 +1284,8 @@ 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;
diff --git a/net/sunrpc/xprtrdma/xprt_rdma.h b/net/sunrpc/xprtrdma/xprt_rdma.h
index 93f3a36..f9ac1fd 100644
--- a/net/sunrpc/xprtrdma/xprt_rdma.h
+++ b/net/sunrpc/xprtrdma/xprt_rdma.h
@@ -93,6 +93,8 @@ enum {
*/
struct rpcrdma_ep {
+ unsigned int rep_send_count;
+ unsigned int rep_send_batch;
atomic_t rep_cqcount;
int rep_cqinit;
int rep_connected;
@@ -229,6 +231,20 @@ struct rpcrdma_rep {
struct ib_recv_wr rr_recv_wr;
};
+/* struct rpcrdma_sendctx - DMA mapped SGEs to unmap after Send completes
+ */
+struct rpcrdma_buffer;
+struct rpcrdma_sendctx {
+ struct ib_send_wr sc_wr;
+ struct ib_cqe sc_cqe;
+
+ struct rpcrdma_buffer *sc_buffers;
+
+ unsigned int sc_unmap_count;
+ struct ib_device *sc_device;
+ struct ib_sge sc_sges[];
+};
+
/*
* struct rpcrdma_mw - external memory region metadata
*
@@ -337,6 +353,16 @@ enum {
RPCRDMA_MAX_SEND_SGES = 1 + 1 + RPCRDMA_MAX_PAGE_SGES + 1,
};
+/* Limit the number of SGEs that can be unmapped during one
+ * Send completion. This caps the amount of work a single
+ * completion can do before returning to the provider.
+ *
+ * Setting this to zero disables Send completion batching.
+ */
+enum {
+ RPCRDMA_MAX_SEND_BATCH = 16,
+};
+
struct rpcrdma_buffer;
struct rpcrdma_req {
struct list_head rl_list;
@@ -400,6 +426,11 @@ struct rpcrdma_buffer {
struct list_head rb_mws;
struct list_head rb_all;
+ unsigned long rb_sc_head;
+ unsigned long rb_sc_tail;
+ unsigned long rb_sc_last;
+ struct rpcrdma_sendctx **rb_sc_ctxs;
+
spinlock_t rb_lock; /* protect buf lists */
int rb_send_count, rb_recv_count;
struct list_head rb_send_bufs;
@@ -455,6 +486,7 @@ struct rpcrdma_stats {
unsigned long mrs_recovered;
unsigned long mrs_orphaned;
unsigned long mrs_allocated;
+ unsigned long empty_sendctx_q;
/* accessed when receiving a reply */
unsigned long long total_rdma_reply;
@@ -556,6 +588,8 @@ struct rpcrdma_rep *rpcrdma_create_rep(struct rpcrdma_xprt *);
void rpcrdma_destroy_req(struct rpcrdma_req *);
int rpcrdma_buffer_create(struct rpcrdma_xprt *);
void rpcrdma_buffer_destroy(struct rpcrdma_buffer *);
+struct rpcrdma_sendctx *rpcrdma_sendctx_get_locked(struct rpcrdma_buffer *buf);
+void rpcrdma_sendctx_put_locked(struct rpcrdma_sendctx *sc);
static inline void
rpcrdma_insert_req(struct rpcrdma_buffer *buffers, struct rpcrdma_req *req)
@@ -645,6 +679,7 @@ int rpcrdma_prepare_send_sges(struct rpcrdma_xprt *r_xprt,
struct xdr_buf *xdr,
enum rpcrdma_chunktype rtype);
void rpcrdma_unmap_sges(struct rpcrdma_ia *, struct rpcrdma_req *);
+void __rpcrdma_unmap_send_sges(struct rpcrdma_sendctx *sc);
int rpcrdma_marshal_req(struct rpcrdma_xprt *r_xprt, struct rpc_rqst *rqst);
void rpcrdma_set_max_header_sizes(struct rpcrdma_xprt *);
void rpcrdma_reply_handler(struct work_struct *work);
@@ -655,6 +690,12 @@ static inline void rpcrdma_set_xdrlen(struct xdr_buf *xdr, size_t len)
xdr->len = len;
}
+static inline void rpcrdma_unmap_send_sges(struct rpcrdma_sendctx *sc)
+{
+ if (unlikely(sc->sc_unmap_count))
+ __rpcrdma_unmap_send_sges(sc);
+}
+
/* RPC/RDMA module init - xprtrdma/transport.c
*/
extern unsigned int xprt_rdma_max_inline_read;
--
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