From 88c7b2b75132c3ff8180b71e4f06cf043a00eac8 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@fb.com>
Date: Thu, 25 Aug 2016 08:07:30 -0600
Subject: [PATCH] blk-mq: prefetch request in blk_mq_tag_to_rq()

When drivers or the core calls this function, they usually
dereference the request shortly there after. Prefetch the first
cache line.

Profiling IO workloads shows that this is the most common cache
miss on the block side of things.

Signed-off-by: Jens Axboe <axboe@fb.com>
---
 block/blk-mq.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index b68fdcbe58f6..eea0d230faa1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -22,6 +22,7 @@
 #include <linux/sched/sysctl.h>
 #include <linux/delay.h>
 #include <linux/crash_dump.h>
+#include <linux/prefetch.h>
 
 #include <trace/events/block.h>
 
@@ -588,8 +589,10 @@ EXPORT_SYMBOL(blk_mq_abort_requeue_list);
 
 struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
 {
-	if (tag < tags->nr_tags)
+	if (tag < tags->nr_tags) {
+		prefetch(tags->rqs[tag]);
 		return tags->rqs[tag];
+	}
 
 	return NULL;
 }
-- 
2.20.1