iser-target: Skip data copy if all the command data comes as immediate

Given that supporting zcopy immediate data for all IOs requires
iser driver to use its own buffer allocations, we settle with
avoiding data copy for IOs with data length of up to 8K (which
is more latency sensitive anyway).

This trims IO write latency by up to 3us and increase IOPs
by up to 40% by saving CPU time doing sg_copy_from_buffer
(8K IO size is the obvious winner here).

Signed-off-by: Jenny Derzhavetz <jennyf@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

1 files changed, 18 insertions, 8 deletions

diff --git a/drivers/infiniband/ulp/isert/ib_isert.c b/drivers/infiniband/ulp/isert/ib_isert.c
index 233b8c7a78df..aa59037d7504 100644
--- a/drivers/infiniband/ulp/isert/ib_isert.c
+++ b/drivers/infiniband/ulp/isert/ib_isert.c
@@ -1315,9 +1315,9 @@ isert_handle_scsi_cmd(struct isert_conn *isert_conn,
 {
 	struct iscsi_conn *conn = isert_conn->conn;
 	struct iscsi_scsi_req *hdr = (struct iscsi_scsi_req *)buf;
-	struct scatterlist *sg;
 	int imm_data, imm_data_len, unsol_data, sg_nents, rc;
 	bool dump_payload = false;
+	unsigned int data_len;
 
 	rc = iscsit_setup_scsi_cmd(conn, cmd, buf);
 	if (rc < 0)
@@ -1326,7 +1326,10 @@ isert_handle_scsi_cmd(struct isert_conn *isert_conn,
 	imm_data = cmd->immediate_data;
 	imm_data_len = cmd->first_burst_len;
 	unsol_data = cmd->unsolicited_data;
+	data_len = cmd->se_cmd.data_length;
 
+	if (imm_data && imm_data_len == data_len)
+		cmd->se_cmd.se_cmd_flags |= SCF_PASSTHROUGH_SG_TO_MEM_NOALLOC;
 	rc = iscsit_process_scsi_cmd(conn, cmd, hdr);
 	if (rc < 0) {
 		return 0;
@@ -1338,13 +1341,20 @@ isert_handle_scsi_cmd(struct isert_conn *isert_conn,
 	if (!imm_data)
 		return 0;
 
-	sg = &cmd->se_cmd.t_data_sg[0];
-	sg_nents = max(1UL, DIV_ROUND_UP(imm_data_len, PAGE_SIZE));
-
-	isert_dbg("Copying Immediate SG: %p sg_nents: %u from %p imm_data_len: %d\n",
-		  sg, sg_nents, &rx_desc->data[0], imm_data_len);
-
-	sg_copy_from_buffer(sg, sg_nents, &rx_desc->data[0], imm_data_len);
+	if (imm_data_len != data_len) {
+		sg_nents = max(1UL, DIV_ROUND_UP(imm_data_len, PAGE_SIZE));
+		sg_copy_from_buffer(cmd->se_cmd.t_data_sg, sg_nents,
+				    &rx_desc->data[0], imm_data_len);
+		isert_dbg("Copy Immediate sg_nents: %u imm_data_len: %d\n",
+			  sg_nents, imm_data_len);
+	} else {
+		sg_init_table(&isert_cmd->sg, 1);
+		cmd->se_cmd.t_data_sg = &isert_cmd->sg;
+		cmd->se_cmd.t_data_nents = 1;
+		sg_set_buf(&isert_cmd->sg, &rx_desc->data[0], imm_data_len);
+		isert_dbg("Transfer Immediate imm_data_len: %d\n",
+			  imm_data_len);
+	}
 
 	cmd->write_data_done += imm_data_len;