From: Bart Van Assche <bvanassche@acm.org>
Date: Wed, 18 Dec 2024 21:22:45 +0000 (-0800)
Subject: block: Reorder the request allocation code in blk_mq_submit_bio()
X-Git-Tag: v6.14-rc1~202^2~7
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=44e41381591dc5b4ea67a9f170b4ec85c817586e;p=thirdparty%2Fkernel%2Flinux.git

block: Reorder the request allocation code in blk_mq_submit_bio()

Help the CPU branch predictor in case of a cache hit by handling the cache
hit scenario first.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20241218212246.1073149-2-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 655dcc16db769..b815ac4872b7d 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3125,12 +3125,12 @@ void blk_mq_submit_bio(struct bio *bio)
 		goto queue_exit;
 
 new_request:
-	if (!rq) {
+	if (rq) {
+		blk_mq_use_cached_rq(rq, plug, bio);
+	} else {
 		rq = blk_mq_get_new_requests(q, plug, bio, nr_segs);
 		if (unlikely(!rq))
 			goto queue_exit;
-	} else {
-		blk_mq_use_cached_rq(rq, plug, bio);
 	}
 
 	trace_block_getrq(bio);