[PATCH 13/14] nvmet: use minimized version of blk_rq_append_bio

[Chaitanya Kulkarni]

The function blk_rq_append_bio() is a genereric API written for all
types driver (having bounce buffers) and different context (where
request is already having a bio i.e. rq->bio != NULL).

It does mainly three things: calculating the segments, bounce queue and
if req->bio == NULL call blk_rq_bio_prep() or handle low level merge()
case.

The NVMe PCIe driver does not use the queue bounce mechanism. In order
to find this out for each request processing in the passthru
blk_rq_append_bio() does extra work in the fast path for each request.

When I ran I/Os with different block sizes on the passthru controller
I found that we can reuse the req->sg_cnt instead of iterating over the
bvecs to find out nr_segs in blk_rq_append_bio(). This calculation in
blk_rq_append_bio() is a duplication of work given that we have the
value in req->sg_cnt. (correct me here if I'm wrong).

With NVMe passthru request based driver we allocate fresh request each
time, so every call to blk_rq_append_bio() rq->bio will be NULL i.e.
we don't really need the second condition in the blk_rq_append_bio()
and the resulting error condition in the caller of blk_rq_append_bio().

So for NVMeOF passthru driver recalculating the segments, bounce check
and ll_back_merge_code is not needed such that we can get away with the
minimal version of the blk_rq_append_bio() which removes the error check
in the fast path along with extra variable in nvmet_passthru_map_sg().

This patch updates the nvmet_passthru_map_sg() such that it does only
appending the bio to the request in the context of the NVMeOF Passthru
driver. Following are perf numbers :-

With current implementation (blk_rq_append_bio()) :-
----------------------------------------------------
+    5.80%     0.02%  kworker/0:2-mm_  [nvmet]  [k] nvmet_passthru_execute_cmd
+    5.44%     0.01%  kworker/0:2-mm_  [nvmet]  [k] nvmet_passthru_execute_cmd
+    4.88%     0.00%  kworker/0:2-mm_  [nvmet]  [k] nvmet_passthru_execute_cmd
+    5.44%     0.01%  kworker/0:2-mm_  [nvmet]  [k] nvmet_passthru_execute_cmd
+    4.86%     0.01%  kworker/0:2-mm_  [nvmet]  [k] nvmet_passthru_execute_cmd
+    5.17%     0.00%  kworker/0:2-eve  [nvmet]  [k] nvmet_passthru_execute_cmd

With this patch :-
----------------------------------------------------
+    3.14%     0.02%  kworker/0:2-eve  [nvmet]  [k] nvmet_passthru_execute_cmd
+    3.26%     0.01%  kworker/0:2-eve  [nvmet]  [k] nvmet_passthru_execute_cmd
+    5.37%     0.01%  kworker/0:2-mm_  [nvmet]  [k] nvmet_passthru_execute_cmd
+    5.18%     0.02%  kworker/0:2-eve  [nvmet]  [k] nvmet_passthru_execute_cmd
+    4.84%     0.02%  kworker/0:2-mm_  [nvmet]  [k] nvmet_passthru_execute_cmd
+    4.87%     0.01%  kworker/0:2-mm_  [nvmet]  [k] nvmet_passthru_execute_cmd

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni at wdc.com>
---
 drivers/nvme/target/passthru.c | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/drivers/nvme/target/passthru.c b/drivers/nvme/target/passthru.c
index 5925e93c4682..a7ccc817ac2d 100644
--- a/drivers/nvme/target/passthru.c
+++ b/drivers/nvme/target/passthru.c
@@ -9,6 +9,7 @@
  */
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 #include <linux/module.h>
+#include <linux/blk-mq.h>
 
 #include "../host/nvme.h"
 #include "nvmet.h"
@@ -205,7 +206,7 @@ static int nvmet_passthru_map_sg(struct nvmet_req *req, struct request *rq)
 	int sg_cnt = req->sg_cnt;
 	struct scatterlist *sg;
 	struct bio *bio;
-	int i, ret;
+	int i;
 
 	if (nvme_is_write(req->cmd)) {
 		op_flags = REQ_SYNC | REQ_IDLE;
@@ -231,11 +232,7 @@ static int nvmet_passthru_map_sg(struct nvmet_req *req, struct request *rq)
 		sg_cnt--;
 	}
 
-	ret = blk_rq_append_bio(rq, &bio);
-	if (unlikely(ret)) {
-		bio_put(bio);
-		return ret;
-	}
+	blk_rq_bio_prep(rq, bio, req->sg_cnt);
 
 	return 0;
 }
-- 
2.22.1