[PATCH 2/3] block: align nr_active_requests_shared_tags to avoid cache line contention

[Sumit Saxena <sumit.saxena@broadcom.com>]

From: James Rizzo <james.rizzo@broadcom.com>

Place nr_active_requests_shared_tags on its own cache line so it does not
share a cache line with nr_requests and other hot fields, avoiding
significant performance hits from false sharing on some CPU architectures.

Signed-off-by: James Rizzo <james.rizzo@broadcom.com>
Signed-off-by: Sumit Saxena <sumit.saxena@broadcom.com>
---
 include/linux/blkdev.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index d463b9b5a0a5..7ed566c81c1b 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -561,7 +561,9 @@ struct request_queue {
 	struct timer_list	timeout;
 	struct work_struct	timeout_work;
 
-	atomic_t		nr_active_requests_shared_tags;
+	/* ensure nr_active_requests_shared_tags and nr_requests are on different cache lines
+	   to avoid significant performance hits on cache line contention on some CPU architectures */
+	atomic_t		nr_active_requests_shared_tags ____cacheline_aligned_in_smp;
 
 	struct blk_mq_tags	*sched_shared_tags;
 
-- 
2.43.7