[v2,5/6] scsi_io_completion hints on fastpatch

Commit Message

Douglas Gilbert March 18, 2018, 8:59 p.m. UTC

Add likely() and unlikely() hints to conditionals on or near the fastpath.

Signed-off-by: Douglas Gilbert <[e-mail]>
Reviewed-by: Johannes Thumshirn <[e-mail]>
---
A reviewer wanted any performance improvement (or otherwise) quantified.
The improvement was so small, that ftrace ignored it. Inline timing code
suggests the improvement from this whole patchset is around 7 nanoseconds
per invocation (tested on a Lenovo X270 (i5-7200U)). Not exactly huge.
Another win might be the smaller size of scsi_io_completion() after the
refactoring; this should allow more other code to fit in the instruction
cache.

 drivers/scsi/scsi_lib.c | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

Comments

Bart Van Assche March 27, 2018, 12:16 a.m. UTC | #1

On Sun, 2018-03-18 at 21:59 +0100, Douglas Gilbert wrote:
> -	/*

> -	 * Kill remainder if no retrys.

> -	 */

> -	if (blk_stat && scsi_noretry_cmd(cmd)) {

> +	/* Kill remainder if no retrys  */


Since you are modifying this comment please also fix the spelling in that
comment ("retrys" -> "retries"). Since otherwise this patch looks fine to me:

Reviewed-by: Bart Van Assche <[e-mail]>

Patch

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 0e2dcdb63fcd..63f79b0dbbf3 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1020,7 +1020,7 @@  void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 	struct request *req = cmd->request;
 	blk_status_t blk_stat = BLK_STS_OK;	/* u8: BLK_STS_OK is only 0 */
 
-	if (result) {	/* does not necessarily mean there is an error */
+	if (unlikely(result)) {	/* a nz result may or may not be an error */
 		blk_stat = scsi_io_completion_nz_result(cmd, result);
 		if (blk_stat == BLK_STS_OK)
 			result = 0;	/* suppress error processing now */
@@ -1033,14 +1033,14 @@  void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 
 	}
 
-	if (blk_rq_is_passthrough(req)) {
+	if (unlikely(blk_rq_is_passthrough(req))) {
 		/*
 		 * __scsi_error_from_host_byte may have reset the host_byte
 		 */
 		scsi_req(req)->result = cmd->result;
 		scsi_req(req)->resid_len = scsi_get_resid(cmd);
 
-		if (scsi_bidi_cmnd(cmd)) {
+		if (unlikely(scsi_bidi_cmnd(cmd))) {
 			/*
 			 * Bidi commands Must be complete as a whole,
 			 * both sides at once.
@@ -1053,7 +1053,7 @@  void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 		}
 	}
 
-	/* no bidi support for !blk_rq_is_passthrough yet */
+	/* no bidi support yet, other than in pass-through */
 	BUG_ON(blk_bidi_rq(req));
 
 	/*
@@ -1069,15 +1069,13 @@  void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 	 * handle. Failed, zero length commands always need to drop down
 	 * to retry code. Fast path should return in this block.
 	 */
-	if (blk_rq_bytes(req) > 0 || blk_stat == BLK_STS_OK) {
-		if (!scsi_end_request(req, blk_stat, good_bytes, 0))
-			return; /* no bytes remaining */
+	if (likely(blk_rq_bytes(req) > 0 || blk_stat == BLK_STS_OK)) {
+		if (likely(!scsi_end_request(req, blk_stat, good_bytes, 0)))
+			return;	/* no bytes remaining */
 	}
 
-	/*
-	 * Kill remainder if no retrys.
-	 */
-	if (blk_stat && scsi_noretry_cmd(cmd)) {
+	/* Kill remainder if no retrys  */
+	if (unlikely(blk_stat && scsi_noretry_cmd(cmd))) {
 		if (scsi_end_request(req, blk_stat, blk_rq_bytes(req), 0))
 			BUG();
 		return;
@@ -1087,14 +1085,14 @@  void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 	 * If there had been no error, but we have leftover bytes in the
 	 * requeues just queue the command up again.
 	 */
-	if (result == 0) {
+	if (likely(result == 0)) {
 		/*
 		 * Unprep the request and put it back at the head of the
 		 * queue. A new command will be prepared and issued.
 		 * This block is the same as case ACTION_REPREP in
 		 * scsi_io_completion_action() above.
 		 */
-		if (q->mq_ops)
+		if (likely(q->mq_ops))
 			scsi_mq_requeue_cmd(cmd);
 		else {
 			scsi_release_buffers(cmd);