svcrdma: Avoid DMA mapping small RPC Replies

On some platforms, DMA mapping part of a page is more costly than
copying bytes. Indeed, not involving the I/O MMU can help the
RPC/RDMA transport scale better for tiny I/Os across more RDMA
devices. This is because interaction with the I/O MMU is eliminated
for each of these small I/Os. Without the explicit unmapping, the
NIC no longer needs to do a costly internal TLB shoot down for
buffers that are just a handful of bytes.

Since pull-up is now a more a frequent operation, I've introduced a
trace point in the pull-up path. It can be used for debugging or
user-space tools that count pull-up frequency.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/include/trace/events/rpcrdma.h b/include/trace/events/rpcrdma.h
index 74b68547eefb..9238d233f8cf 100644
--- a/include/trace/events/rpcrdma.h
+++ b/include/trace/events/rpcrdma.h
@@ -1639,6 +1639,24 @@ TRACE_EVENT(svcrdma_dma_map_rwctx,
 	)
 );
 
+TRACE_EVENT(svcrdma_send_pullup,
+	TP_PROTO(
+		unsigned int len
+	),
+
+	TP_ARGS(len),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, len)
+	),
+
+	TP_fast_assign(
+		__entry->len = len;
+	),
+
+	TP_printk("len=%u", __entry->len)
+);
+
 TRACE_EVENT(svcrdma_send_failed,
 	TP_PROTO(
 		const struct svc_rqst *rqst,