svcrdma's current SQ accounting algorithm takes sc_lock and disables
bottom-halves while posting all RDMA Read, Write, and Send WRs.
This is relatively heavyweight serialization. And note that Write and
Send are already fully serialized by the xpt_mutex.
Using a single atomic_t should be all that is necessary to guarantee
that ib_post_send() is called only when there is enough space on the
send queue. This is what the other RDMA-enabled storage targets do.
Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
---
include/linux/sunrpc/svc_rdma.h | 2 +-
net/sunrpc/xprtrdma/svc_rdma_sendto.c | 7 ++++++-
net/sunrpc/xprtrdma/svc_rdma_transport.c | 18 +++++++-----------
3 files changed, 14 insertions(+), 13 deletions(-)
diff --git a/include/linux/sunrpc/svc_rdma.h b/include/linux/sunrpc/svc_rdma.h
index 6aef63b..601cb07 100644
--- a/include/linux/sunrpc/svc_rdma.h
+++ b/include/linux/sunrpc/svc_rdma.h
@@ -139,7 +139,7 @@ struct svcxprt_rdma {
int sc_max_sge_rd; /* max sge for read target */
bool sc_snd_w_inv; /* OK to use Send With Invalidate */
- atomic_t sc_sq_count; /* Number of SQ WR on queue */
+ atomic_t sc_sq_avail; /* SQEs ready to be consumed */
unsigned int sc_sq_depth; /* Depth of SQ */
unsigned int sc_rq_depth; /* Depth of RQ */
u32 sc_max_requests; /* Forward credits */
diff --git a/net/sunrpc/xprtrdma/svc_rdma_sendto.c b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
index 0a58d40..30eeab5 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_sendto.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_sendto.c
@@ -594,7 +594,12 @@ int svc_rdma_sendto(struct svc_rqst *rqstp)
goto err0;
inline_bytes = rqstp->rq_res.len;
- /* Create the RDMA response header */
+ /* Create the RDMA response header. xprt->xpt_mutex,
+ * acquired in svc_send(), serializes RPC replies. The
+ * code path below that inserts the credit grant value
+ * into each transport header runs only inside this
+ * critical section.
+ */
ret = -ENOMEM;
res_page = alloc_page(GFP_KERNEL);
if (!res_page)
diff --git a/net/sunrpc/xprtrdma/svc_rdma_transport.c b/net/sunrpc/xprtrdma/svc_rdma_transport.c
index 6864fb9..f7dd6494 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_transport.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_transport.c
@@ -434,7 +434,7 @@ static void svc_rdma_send_wc_common(struct svcxprt_rdma *xprt,
goto err;
out:
- atomic_dec(&xprt->sc_sq_count);
+ atomic_inc(&xprt->sc_sq_avail);
wake_up(&xprt->sc_send_wait);
return;
@@ -1008,6 +1008,7 @@ static struct svc_xprt *svc_rdma_accept(struct svc_xprt *xprt)
newxprt->sc_rq_depth = newxprt->sc_max_requests +
newxprt->sc_max_bc_requests;
newxprt->sc_sq_depth = RPCRDMA_SQ_DEPTH_MULT * newxprt->sc_rq_depth;
+ atomic_set(&newxprt->sc_sq_avail, newxprt->sc_sq_depth);
if (!svc_rdma_prealloc_ctxts(newxprt))
goto errout;
@@ -1333,15 +1334,13 @@ int svc_rdma_send(struct svcxprt_rdma *xprt, struct ib_send_wr *wr)
/* If the SQ is full, wait until an SQ entry is available */
while (1) {
- spin_lock_bh(&xprt->sc_lock);
- if (xprt->sc_sq_depth < atomic_read(&xprt->sc_sq_count) + wr_count) {
- spin_unlock_bh(&xprt->sc_lock);
+ if ((atomic_sub_return(wr_count, &xprt->sc_sq_avail) < 0)) {
atomic_inc(&rdma_stat_sq_starve);
/* Wait until SQ WR available if SQ still full */
+ atomic_add(wr_count, &xprt->sc_sq_avail);
wait_event(xprt->sc_send_wait,
- atomic_read(&xprt->sc_sq_count) <
- xprt->sc_sq_depth);
+ atomic_read(&xprt->sc_sq_avail) > wr_count);
if (test_bit(XPT_CLOSE, &xprt->sc_xprt.xpt_flags))
return -ENOTCONN;
continue;
@@ -1351,19 +1350,16 @@ int svc_rdma_send(struct svcxprt_rdma *xprt, struct ib_send_wr *wr)
svc_xprt_get(&xprt->sc_xprt);
/* Bump used SQ WR count and post */
- atomic_add(wr_count, &xprt->sc_sq_count);
ret = ib_post_send(xprt->sc_qp, wr, &bad_wr);
if (ret) {
set_bit(XPT_CLOSE, &xprt->sc_xprt.xpt_flags);
- atomic_sub(wr_count, &xprt->sc_sq_count);
for (i = 0; i < wr_count; i ++)
svc_xprt_put(&xprt->sc_xprt);
dprintk("svcrdma: failed to post SQ WR rc=%d, "
- "sc_sq_count=%d, sc_sq_depth=%d\n",
- ret, atomic_read(&xprt->sc_sq_count),
+ "sc_sq_avail=%d, sc_sq_depth=%d\n",
+ ret, atomic_read(&xprt->sc_sq_avail),
xprt->sc_sq_depth);
}
- spin_unlock_bh(&xprt->sc_lock);
if (ret)
wake_up(&xprt->sc_send_wait);
break;
--
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