rpcrdma_buffer_get acquires an rpcrdma_req and rep for each RPC.
Currently this is done in the call_allocate action, and sometimes it
can fail if there are many outstanding RPCs.
When call_allocate fails, the RPC task is put on the delayq. It is
awoken a few milliseconds later, but there's no guarantee it will
get a buffer at that time. The RPC task can be repeatedly put back
to sleep or even starved.
The call_allocate action should rarely fail. The delayq mechanism is
not meant to deal with transport congestion.
In the current sunrpc stack, there is a friendlier way to deal with
this situation. These objects are actually tantamount to an RPC
slot (rpc_rqst) and there is a separate FSM action, distinct from
call_allocate, for allocating slot resources. This is the
call_reserve action.
When allocation fails during this action, the RPC is placed on the
transport's backlog queue. The backlog mechanism provides a stronger
guarantee that when the RPC is awoken, a buffer will be available
for it; and backlogged RPCs are awoken one-at-a-time.
To make slot resource allocation occur in the call_reserve action,
create special ->alloc_slot and ->free_slot call-outs for xprtrdma.
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---
net/sunrpc/xprtrdma/transport.c | 52 ++++++++++++++++++++++++++++++++++++++-
1 file changed, 50 insertions(+), 2 deletions(-)
diff --git a/net/sunrpc/xprtrdma/transport.c b/net/sunrpc/xprtrdma/transport.c
index cf5e866..8f9338e 100644
--- a/net/sunrpc/xprtrdma/transport.c
+++ b/net/sunrpc/xprtrdma/transport.c
@@ -538,6 +538,54 @@
}
}
+/**
+ * xprt_rdma_alloc_slot - allocate an rpc_rqst
+ * @xprt: controlling RPC transport
+ * @task: RPC task requesting a fresh rpc_rqst
+ *
+ * tk_status values:
+ * %0 if task->tk_rqstp points to a fresh rpc_rqst
+ * %-EAGAIN if no rpc_rqst is available; queued on backlog
+ */
+static void
+xprt_rdma_alloc_slot(struct rpc_xprt *xprt, struct rpc_task *task)
+{
+ struct rpc_rqst *rqst;
+
+ spin_lock(&xprt->reserve_lock);
+ if (list_empty(&xprt->free))
+ goto out_sleep;
+ rqst = list_first_entry(&xprt->free, struct rpc_rqst, rq_list);
+ list_del(&rqst->rq_list);
+ spin_unlock(&xprt->reserve_lock);
+
+ task->tk_rqstp = rqst;
+ task->tk_status = 0;
+ return;
+
+out_sleep:
+ rpc_sleep_on(&xprt->backlog, task, NULL);
+ spin_unlock(&xprt->reserve_lock);
+ task->tk_status = -EAGAIN;
+}
+
+/**
+ * xprt_rdma_free_slot - release an rpc_rqst
+ * @xprt: controlling RPC transport
+ * @rqst: rpc_rqst to release
+ *
+ */
+static void
+xprt_rdma_free_slot(struct rpc_xprt *xprt, struct rpc_rqst *rqst)
+{
+ memset(rqst, 0, sizeof(*rqst));
+
+ spin_lock(&xprt->reserve_lock);
+ list_add(&rqst->rq_list, &xprt->free);
+ rpc_wake_up_next(&xprt->backlog);
+ spin_unlock(&xprt->reserve_lock);
+}
+
static bool
rpcrdma_get_sendbuf(struct rpcrdma_xprt *r_xprt, struct rpcrdma_req *req,
size_t size, gfp_t flags)
@@ -780,8 +828,8 @@ void xprt_rdma_print_stats(struct rpc_xprt *xprt, struct seq_file *seq)
static const struct rpc_xprt_ops xprt_rdma_procs = {
.reserve_xprt = xprt_reserve_xprt_cong,
.release_xprt = xprt_release_xprt_cong, /* sunrpc/xprt.c */
- .alloc_slot = xprt_alloc_slot,
- .free_slot = xprt_free_slot,
+ .alloc_slot = xprt_rdma_alloc_slot,
+ .free_slot = xprt_rdma_free_slot,
.release_request = xprt_release_rqst_cong, /* ditto */
.set_retrans_timeout = xprt_set_retrans_timeout_def, /* ditto */
.timer = xprt_rdma_timer,
--
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