null_blk: set use_per_node_hctx param to false

The defaults for the module is to instantiate itself with blk-mq and a
submit queue for each CPU node in the system.

To save resources, initialize instead with a single submit queue.
Signed-off-by: Matias Bjorling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/block/null_blk.txt

@@ -65,7 +65,8 @@ hw_queue_depth=[0..qdepth]: Default: 64
 
 III: Multi-queue specific parameters
 
-use_per_node_hctx=[0/1]: Default: 1
-  If 1, the multi-queue block layer is instantiated with a hardware dispatch
-  queue for each CPU node in the system. If 0, it is instantiated with the
-  number of queues defined in the submit_queues parameter.
+use_per_node_hctx=[0/1]: Default: 0
+  0: The number of submit queues are set to the value of the submit_queues
+     parameter.
+  1: The multi-queue block layer is instantiated with a hardware dispatch
+     queue for each CPU node in the system.

drivers/block/null_blk.c

@@ -101,9 +101,9 @@ static int hw_queue_depth = 64;
 module_param(hw_queue_depth, int, S_IRUGO);
 MODULE_PARM_DESC(hw_queue_depth, "Queue depth for each hardware queue. Default: 64");
 
-static bool use_per_node_hctx = true;
+static bool use_per_node_hctx = false;
 module_param(use_per_node_hctx, bool, S_IRUGO);
-MODULE_PARM_DESC(use_per_node_hctx, "Use per-node allocation for hardware context queues. Default: true");
+MODULE_PARM_DESC(use_per_node_hctx, "Use per-node allocation for hardware context queues. Default: false");
 
 static void put_tag(struct nullb_queue *nq, unsigned int tag)
 {