[PATCH v3] btrfs: cleanup btrfs_async_submit_limit to return the final limit value

[Anand Jain <anand.jain@oracle.com>]

We feedback IO progress when it falls below the 2/3 times of the limit
obtained from btrfs_async_submit_limit() so to creates a wait for the
write process and make uncontested progress during the async submission.

In general device/transport q depth is 256 and, btrfs_async_submit_limit()
returns 256 times per device which originally was introduced by [1]. But
256 at the device level is for all types of IOs (R/W sync/async) and so
may be it was possible that entire of 256 could have occupied by async
writes and, so later patch [2] took only 2/3 times of 256 which seemed to
work well.

 [1]
 cb03c743c648
 Btrfs: Change the congestion functions to meter the number of async submits as well

 [2]
 4854ddd0ed0a
 Btrfs: Wait for kernel threads to make progress during async submission

This patch is a cleanup patch, no functional changes. And now as we are taking
only 2/3 of limit (256), so btrfs_async_submit_limit() will return 256 * 2/3.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
v2: add more change log.
v3: don't compute 256 * 2/3. I didn't know compiler will do it anyway,
    thats nice. So keeping that open coded. And comment removed.

 fs/btrfs/disk-io.c | 4 ++--
 fs/btrfs/volumes.c | 1 -
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index dfdab849037b..6e27259e965b 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -861,7 +861,8 @@ unsigned long btrfs_async_submit_limit(struct btrfs_fs_info *info)
 	unsigned long limit = min_t(unsigned long,
 				    info->thread_pool_size,
 				    info->fs_devices->open_devices);
-	return 256 * limit;
+
+	return (256 * 2/3) * limit;
 }
 
 static void run_one_async_start(struct btrfs_work *work)
@@ -887,7 +888,6 @@ static void run_one_async_done(struct btrfs_work *work)
 	fs_info = async->fs_info;
 
 	limit = btrfs_async_submit_limit(fs_info);
-	limit = limit * 2 / 3;
 
 	/*
 	 * atomic_dec_return implies a barrier for waitqueue_active
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index b39737568c22..61cefa37b56a 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -376,7 +376,6 @@ static noinline void run_scheduled_bios(struct btrfs_device *device)
 
 	bdi = device->bdev->bd_bdi;
 	limit = btrfs_async_submit_limit(fs_info);
-	limit = limit * 2 / 3;
 
 loop:
 	spin_lock(&device->io_lock);
-- 
2.13.1