[PATCH v2 08/19] svcrdma: Don't account for Receive queue "starvation"

Chuck Lever
2017-06-16T15:21:26Z

>From what I can tell, calling ->recvfrom when there is no work to do
is a normal part of operation. This is the only way svc_recv can
tell when there is no more data ready to receive on the transport.

Neither the TCP nor the UDP transport implementations have a
"starve" metric.

The cost of receive starvation accounting is bumping an atomic, which
results in extra (IMO unnecessary) bus traffic between CPU sockets,
while holding a spin lock.

Signed-off-by: Chuck Lever <chuck.lever-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
---
net/sunrpc/xprtrdma/svc_rdma_recvfrom.c | 21 ++++++---------------
1 file changed, 6 insertions(+), 15 deletions(-)

diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
index b480893..1452bd0 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c
@@ -844,9 +844,9 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
struct svc_xprt *xprt = rqstp->rq_xprt;
struct svcxprt_rdma *rdma_xprt =
container_of(xprt, struct svcxprt_rdma, sc_xprt);
- struct svc_rdma_op_ctxt *ctxt = NULL;
+ struct svc_rdma_op_ctxt *ctxt;
struct rpcrdma_msg *rmsgp;
- int ret = 0;
+ int ret;

dprintk("svcrdma: rqstp=%p\n", rqstp);

@@ -863,21 +863,13 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
struct svc_rdma_op_ctxt, list);
list_del(&ctxt->list);
} else {
- atomic_inc(&rdma_stat_rq_starve);
+ /* No new incoming requests, terminate the loop */
clear_bit(XPT_DATA, &xprt->xpt_flags);
- ctxt = NULL;
+ spin_unlock(&rdma_xprt->sc_rq_dto_lock);
+ return 0;
}
spin_unlock(&rdma_xprt->sc_rq_dto_lock);
- if (!ctxt) {
- /* This is the EAGAIN path. The svc_recv routine will
- * return -EAGAIN, the nfsd thread will go to call into
- * svc_recv again and we shouldn't be on the active
- * transport list
- */
- if (test_bit(XPT_CLOSE, &xprt->xpt_flags))
- goto defer;
- goto out;
- }
+
dprintk("svcrdma: processing ctxt=%p on xprt=%p, rqstp=%p\n",
ctxt, rdma_xprt, rqstp);
atomic_inc(&rdma_stat_recv);
@@ -920,7 +912,6 @@ int svc_rdma_recvfrom(struct svc_rqst *rqstp)
+ rqstp->rq_arg.page_len
+ rqstp->rq_arg.tail[0].iov_len;
svc_rdma_put_context(ctxt, 0);
- out:
dprintk("svcrdma: ret=%d, rq_arg.len=%u, "
"rq_arg.head[0].iov_base=%p, rq_arg.head[0].iov_len=%zd\n",
ret, rqstp->rq_arg.len,