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xprtrdma: Introduce ->alloc_slot call-out for xprtrdma

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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@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
This commit is contained in:
Chuck Lever 2018-05-04 15:35:04 -04:00 committed by Anna Schumaker
parent a9cde23ab7
commit 48be539dd4
1 changed files with 50 additions and 2 deletions
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52
net/sunrpc/xprtrdma/transport.c
View file

@ -538,6 +538,54 @@ xprt_rdma_connect(struct rpc_xprt *xprt, struct rpc_task *task)
}
}
/**
* 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 @@ xprt_rdma_disable_swap(struct rpc_xprt *xprt)
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,