From 6ebfe58117d478a369bea929973f4ac381aa6dea Mon Sep 17 00:00:00 2001
From: Isaac Huang <he.huang@intel.com>
Date: Thu, 29 Sep 2016 14:13:31 -0600
Subject: [PATCH] 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 +++++++++++++++++++++
 config/kernel.m4                  |  1 +
 module/zfs/vdev_disk.c            | 13 +++++++++++++
 3 files changed, 35 insertions(+)

diff --git a/config/kernel-blk-queue-unplug.m4 b/config/kernel-blk-queue-unplug.m4
index 45cc2322a5..075fbccd1a 100644
--- a/config/kernel-blk-queue-unplug.m4
+++ b/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"
+])
diff --git a/config/kernel.m4 b/config/kernel.m4
index 0e892b9c38..fcbb686c27 100644
--- a/config/kernel.m4
+++ b/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_HAVE_BIO_SET_OP_ATTRS
diff --git a/module/zfs/vdev_disk.c b/module/zfs/vdev_disk.c
index 393737377e..5697f68671 100644
--- a/module/zfs/vdev_disk.c
+++ b/module/zfs/vdev_disk.c
@@ -520,6 +520,9 @@ __vdev_disk_physio(struct block_device *bdev, zio_t *zio, caddr_t kbuf_ptr,
 	uint64_t bio_offset;
 	int bio_size, bio_count = 16;
 	int i = 0, error = 0;
+#if defined(HAVE_BLK_QUEUE_HAVE_BLK_PLUG)
+	struct blk_plug plug;
+#endif
 
 	ASSERT3U(kbuf_offset + kbuf_size, <=, bdev->bd_inode->i_size);
 
@@ -590,11 +593,21 @@ retry:
 	if (zio)
 		zio->io_delay = jiffies_64;
 
+#if defined(HAVE_BLK_QUEUE_HAVE_BLK_PLUG)
+	if (dr->dr_bio_count > 1)
+		blk_start_plug(&plug);
+#endif
+
 	/* Submit all bio's associated with this dio */
 	for (i = 0; i < dr->dr_bio_count; i++)
 		if (dr->dr_bio[i])
 			vdev_submit_bio(dr->dr_bio[i]);
 
+#if defined(HAVE_BLK_QUEUE_HAVE_BLK_PLUG)
+	if (dr->dr_bio_count > 1)
+		blk_finish_plug(&plug);
+#endif
+
 	(void) vdev_disk_dio_put(dr);
 
 	return (error);