Explicit block device plugging when submitting multiple BIOs

Without plugging, the default 'noop' scheduler will not merge
the BIOs which are part of a large ZIO.

Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Isaac Huang <he.huang@intel.com>
Closes #5181

config/kernel-blk-queue-unplug.m4

@@ -21,3 +21,24 @@ AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE_HAVE_BIO_RW_UNPLUG], [
 	])
 	EXTRA_KCFLAGS="$tmp_flags"
 ])
+
+AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE_HAVE_BLK_PLUG], [
+	AC_MSG_CHECKING([whether struct blk_plug is available])
+	tmp_flags="$EXTRA_KCFLAGS"
+	EXTRA_KCFLAGS="${NO_UNUSED_BUT_SET_VARIABLE}"
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/blkdev.h>
+	],[
+		struct blk_plug plug;
+
+		blk_start_plug(&plug);
+		blk_finish_plug(&plug);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_BLK_QUEUE_HAVE_BLK_PLUG, 1,
+		          [struct blk_plug is available])
+	],[
+		AC_MSG_RESULT(no)
+	])
+	EXTRA_KCFLAGS="$tmp_flags"
+])

config/kernel.m4

@@ -34,6 +34,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_BLK_QUEUE_MAX_HW_SECTORS
 	ZFS_AC_KERNEL_BLK_QUEUE_MAX_SEGMENTS
 	ZFS_AC_KERNEL_BLK_QUEUE_HAVE_BIO_RW_UNPLUG
+	ZFS_AC_KERNEL_BLK_QUEUE_HAVE_BLK_PLUG
 	ZFS_AC_KERNEL_GET_DISK_RO
 	ZFS_AC_KERNEL_GET_GENDISK
 	ZFS_AC_KERNEL_DISCARD_GRANULARITY