ZEN: INTERACTIVE: Increase block layer queue depth to 512

4.9:
In a surprising turn of events, while benchmarking and testing
hierarchical scheduling with BFQ + writeback throttling, it turns out
that raising the number of requests in queue _actually_ improves
responsiveness and completely eliminates the random stalls that would
normally occur without hierarchical scheduling.

To make this test more intense, I used the following test:

Rotational disk1: rsync -a /source/of/data /target/to/disk1
Rotational disk2: rsync -a /source/of/data /target/to/disk2

And periodically attempted to write super fast with:
dd if=/dev/zero of=/target/to/disk1/block bs=4096

This wrote 10gb incredibly fast to writeback and I encountered zero
stalls through this entire test of 10-15 minutes.

My suspicion is that with cgroups, BFQ is more able to properly sort
among multiple drives, reducing the chance of a starved process.  This
plus writeback throttling completely eliminate any outstanding bugs with
high writeback ratios, letting the user enjoy low latency writes
(application thinks they're already done), and super high throughput due
to batched writes in writeback.

Please note however, without the following configuration, I cannot
guarantee you will not get stalls:

CONFIG_BLK_CGROUP=y
CONFIG_CGROUP_WRITEBACK=y
CONFIG_IOSCHED_CFQ=y
CONFIG_CFQ_GROUP_IOSCHED=y
CONFIG_IOSCHED_BFQ=y
CONFIG_BFQ_GROUP_IOSCHED=y
CONFIG_DEFAULT_BFQ=y
CONFIG_SCSI_MQ_DEFAULT=n

Special thanks to h2, author of smxi and inxi, for providing evidence
that a configuration specific to Debian did not cause stalls found the
Liquorix kernels under heavy IO load.  This specific configuration
turned out to be hierarchical scheduling on CFQ (thus, BFQ as well).

include/linux/blkdev.h

@@ -45,7 +45,11 @@ struct blk_queue_stats;
 struct blk_stat_callback;
 
 #define BLKDEV_MIN_RQ	4
+#ifdef CONFIG_ZEN_INTERACTIVE
+#define BLKDEV_MAX_RQ	512
+#else
 #define BLKDEV_MAX_RQ	128	/* Default maximum */
+#endif
 
 /* Must be consistent with blk_mq_poll_stats_bkt() */
 #define BLK_MQ_POLL_STATS_BKTS 16

init/Kconfig

@@ -82,6 +82,7 @@ config ZEN_INTERACTIVE
 	  --- Block Layer ----------------------------------------
 
 	    Default scheduler.........: mq-deadline -> bfq
+	    Queue depth...............:      128    -> 512
 
 config BROKEN
 	bool