linux-kernel - [PATCH 1/3] blk-mq: optimise rq sort function

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Message-Id: <a21e91fff1abe201ebbc010d596b034f7b5123de.1574974577.git.asml.silence@gmail.com>
Date:   Fri, 29 Nov 2019 00:11:53 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/3] blk-mq: optimise rq sort function

Check "!=" in multi-layer comparisons. The same memory usage, fewer
instructions, and 2 from 4 jumps are replaced with SETcc.

Note, that list_sort() doesn't differ 0 and <0.

Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 block/blk-mq.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 323c9cb28066..f32a3cfdd34e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1668,14 +1668,10 @@ static int plug_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
 	struct request *rqa = container_of(a, struct request, queuelist);
 	struct request *rqb = container_of(b, struct request, queuelist);
 
-	if (rqa->mq_ctx < rqb->mq_ctx)
-		return -1;
-	else if (rqa->mq_ctx > rqb->mq_ctx)
-		return 1;
-	else if (rqa->mq_hctx < rqb->mq_hctx)
-		return -1;
-	else if (rqa->mq_hctx > rqb->mq_hctx)
-		return 1;
+	if (rqa->mq_ctx != rqb->mq_ctx)
+		return rqa->mq_ctx > rqb->mq_ctx;
+	if (rqa->mq_hctx != rqb->mq_hctx)
+		return rqa->mq_hctx > rqb->mq_hctx;
 
 	return blk_rq_pos(rqa) > blk_rq_pos(rqb);
 }
-- 
2.24.0