[PATCH 2/2] loop: Add the default_queue_depth kernel module parameter

[Bart Van Assche <bvanassche@acm.org>]

Recent versions of Android use the zram driver on top of the loop driver.
There is a mismatch between the default loop driver queue depth (128) and
the queue depth of the storage device in my test setup (32). That mismatch
results in write latencies that are higher than necessary. Address this
issue by making the default loop driver queue depth configurable. Compared
to configuring the queue depth by writing into the nr_requests sysfs
attribute, this approach does not involve calling synchronize_rcu() to
modify the queue depth.

Cc: Ming Lei <ming.lei@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Martijn Coenen <maco@android.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 drivers/block/loop.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 9fca3ab3988d..0f1f1ecd941a 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -2098,6 +2098,9 @@ module_param(max_loop, int, 0444);
 MODULE_PARM_DESC(max_loop, "Maximum number of loop devices");
 module_param(max_part, int, 0444);
 MODULE_PARM_DESC(max_part, "Maximum number of partitions per loop device");
+static uint32_t default_queue_depth = 128;
+module_param(default_queue_depth, uint, 0644);
+MODULE_PARM_DESC(default_queue_depth, "Default loop device queue depth");
 MODULE_LICENSE("GPL");
 MODULE_ALIAS_BLOCKDEV_MAJOR(LOOP_MAJOR);
 
@@ -2330,7 +2333,7 @@ static int loop_add(int i)
 	err = -ENOMEM;
 	lo->tag_set.ops = &loop_mq_ops;
 	lo->tag_set.nr_hw_queues = 1;
-	lo->tag_set.queue_depth = 128;
+	lo->tag_set.queue_depth = max(default_queue_depth, 2U);
 	lo->tag_set.numa_node = NUMA_NO_NODE;
 	lo->tag_set.cmd_size = sizeof(struct loop_cmd);
 	lo->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_STACKING |