The read policy type latency routes the read IO based on the historical
average wait-time experienced by the read IOs through the individual
device. This patch obtains the historical read IO stats from the kernel
block layer and calculates its average.

Example usage:
  echo "latency" > /sys/fs/btrfs/$uuid/read_policy

Signed-off-by: Anand Jain <redacted>
---
v3: The block layer commit 0d02129e76ed (block: merge struct block_device and
     struct hd_struct) has changed the first argument in the function
     part_stat_read_all() in 5.11-rc1. So the compilation will fail. This patch
     fixes it.
     Commit log updated.

v2: Use btrfs_debug_rl() instead of btrfs_info_rl()
     It is better we have this debug until we test this on at least few
     hardwares.
     Drop the unrelated changes.
     Update change log.

v1: Drop part_stat_read_all instead use part_stat_read
     Drop inflight


  fs/btrfs/sysfs.c   |  3 ++-
  fs/btrfs/volumes.c | 38 ++++++++++++++++++++++++++++++++++++++
  fs/btrfs/volumes.h |  2 ++
  3 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
index 19b9fffa2c9c..96ca7bef6357 100644
--- a/fs/btrfs/sysfs.c
+++ b/fs/btrfs/sysfs.c

@@ -915,7 +915,8 @@ static bool strmatch(const char *buffer, const char *string)
  	return false;
  }
  
-static const char * const btrfs_read_policy_name[] = { "pid" };
+/* Must follow the order as in enum btrfs_read_policy */
+static const char * const btrfs_read_policy_name[] = { "pid", "latency" };
  
  static ssize_t btrfs_read_policy_show(struct kobject *kobj,
  				      struct kobj_attribute *a, char *buf)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index f4037a6bd926..f7a0a83d2cd4 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c

@@ -14,6 +14,7 @@
  #include <linux/semaphore.h>
  #include <linux/uuid.h>
  #include <linux/list_sort.h>
+#include <linux/part_stat.h>
  #include "misc.h"
  #include "ctree.h"
  #include "extent_map.h"

@@ -5490,6 +5491,39 @@ int btrfs_is_parity_mirror(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
  	return ret;
  }
  
+static int btrfs_find_best_stripe(struct btrfs_fs_info *fs_info,
+				  struct map_lookup *map, int first,
+				  int num_stripe)
+{
+	u64 est_wait = 0;
+	int best_stripe = 0;
+	int index;
+
+	for (index = first; index < first + num_stripe; index++) {
+		u64 read_wait;
+		u64 avg_wait = 0;
+		unsigned long read_ios;
+		struct btrfs_device *device = map->stripes[index].dev;
+
+		read_wait = part_stat_read(device->bdev, nsecs[READ]);
+		read_ios = part_stat_read(device->bdev, ios[READ]);
+
+		if (read_wait && read_ios && read_wait >= read_ios)
+			avg_wait = div_u64(read_wait, read_ios);
+		else
+			btrfs_debug_rl(device->fs_devices->fs_info,