Add likely() and unlikely() hints to conditionals on or near the fastpath.
Signed-off-by: Douglas Gilbert <dgilbert@xxxxxxxxxxxx>
Reviewed-by: Johannes Thumshirn <jthumshirn@xxxxxxx>
---
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 | 22 ++++++++++------------
1 file changed, 10 insertions(+), 12 deletions(-)
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 7bf7d3c4b9c2..a9676d824452 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1028,17 +1028,17 @@ 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;
- if (result) /* does not necessarily mean there is an error */
+ if (unlikely(result)) /* a nz result may or may not be an error */
result = scsi_io_completion_nz_result(cmd, result, &blk_stat);
- 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.
@@ -1051,7 +1051,7 @@ 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));
/*
@@ -1067,15 +1067,13 @@ 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 retries */
+ if (unlikely(blk_stat && scsi_noretry_cmd(cmd))) {
if (scsi_end_request(req, blk_stat, blk_rq_bytes(req), 0))
BUG();
return;
@@ -1085,7 +1083,7 @@ 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))
scsi_io_completion_reprep(cmd, q);
else
scsi_io_completion_action(cmd, result);
--
2.14.1
