net/smc: rdma write inline if qp has sufficient inline space

Rdma write with inline flag when sending small packages,
whose length is shorter than the qp's max_inline_data, can
help reducing latency.

In my test environment, which are 2 VMs running on the same
physical host and whose NICs(ConnectX-4Lx) are working on
SR-IOV mode, qperf shows 0.5us-0.7us improvement in latency.

Test command:
server: smc_run taskset -c 1 qperf
client: smc_run taskset -c 1 qperf <server ip> -oo \
		msg_size:1:2K:*2 -t 30 -vu tcp_lat

The results shown below:
msgsize     before       after
1B          11.2 us      10.6 us (-0.6 us)
2B          11.2 us      10.7 us (-0.5 us)
4B          11.3 us      10.7 us (-0.6 us)
8B          11.2 us      10.6 us (-0.6 us)
16B         11.3 us      10.7 us (-0.6 us)
32B         11.3 us      10.6 us (-0.7 us)
64B         11.2 us      11.2 us (0 us)
128B        11.2 us      11.2 us (0 us)
256B        11.2 us      11.2 us (0 us)
512B        11.4 us      11.3 us (-0.1 us)
1KB         11.4 us      11.5 us (0.1 us)
2KB         11.5 us      11.5 us (0 us)
Signed-off-by: Guangguan Wang <guangguan.wang@linux.alibaba.com>
Reviewed-by: Tony Lu <tonylu@linux.alibaba.com>
Tested-by: kernel test robot <lkp@intel.com>
Acked-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/smc/smc_tx.c

@@ -391,12 +391,20 @@ static int smcr_tx_rdma_writes(struct smc_connection *conn, size_t len,
 	int rc;
 
 	for (dstchunk = 0; dstchunk < 2; dstchunk++) {
-		struct ib_sge *sge =
-			wr_rdma_buf->wr_tx_rdma[dstchunk].wr.sg_list;
+		struct ib_rdma_wr *wr = &wr_rdma_buf->wr_tx_rdma[dstchunk];
+		struct ib_sge *sge = wr->wr.sg_list;
+		u64 base_addr = dma_addr;
+
+		if (dst_len < link->qp_attr.cap.max_inline_data) {
+			base_addr = (uintptr_t)conn->sndbuf_desc->cpu_addr;
+			wr->wr.send_flags |= IB_SEND_INLINE;
+		} else {
+			wr->wr.send_flags &= ~IB_SEND_INLINE;
+		}
 
 		num_sges = 0;
 		for (srcchunk = 0; srcchunk < 2; srcchunk++) {
-			sge[srcchunk].addr = dma_addr + src_off;
+			sge[srcchunk].addr = base_addr + src_off;
 			sge[srcchunk].length = src_len;
 			num_sges++;
 
@@ -410,8 +418,7 @@ static int smcr_tx_rdma_writes(struct smc_connection *conn, size_t len,
 			src_len = dst_len - src_len; /* remainder */
 			src_len_sum += src_len;
 		}
-		rc = smc_tx_rdma_write(conn, dst_off, num_sges,
-				       &wr_rdma_buf->wr_tx_rdma[dstchunk]);
+		rc = smc_tx_rdma_write(conn, dst_off, num_sges, wr);
 		if (rc)
 			return rc;
 		if (dst_len_sum == len)