On Tue, Dec 03, 2019 at 02:42:50PM +0800, Qu Wenruo wrote:
> [PROBLEM]
> There are quite some users reporting that 'btrfs balance cancel' slow to
> cancel current running balance, or even doesn't work for certain dead
> balance loop.
> 
> With the following script showing how long it takes to fully stop a
> balance:
>   #!/bin/bash
>   dev=/dev/test/test
>   mnt=/mnt/btrfs
> 
>   umount $mnt &> /dev/null
>   umount $dev &> /dev/null
> 
>   mkfs.btrfs -f $dev
>   mount $dev -o nospace_cache $mnt
> 
>   dd if=/dev/zero bs=1M of=$mnt/large &
>   dd_pid=$!
> 
>   sleep 3
>   kill -KILL $dd_pid
>   sync
> 
>   btrfs balance start --bg --full $mnt &
>   sleep 1
> 
>   echo "cancel request" >> /dev/kmsg
>   time btrfs balance cancel $mnt
>   umount $mnt
> 
> It takes around 7~10s to cancel the running balance in my test
> environment.
> 
> [CAUSE]
> Btrfs uses btrfs_fs_info::balance_cancel_req to record how many cancel
> request are queued.
> However that cancelling request is only checked after relocating a block
> group.
> 
> That behavior is far from optimal to provide a faster cancelling.
> 
> [FIX]
> This patchset will add more cancelling check points, to make cancelling
> faster.

Nice!  I look forward to using this in the future!

Does this cover device delete/resize as well?  I think there needs to be
a check added for fatal signals for those to work, as they don't respond
to balance cancel.

> And also, introduce a new error injection points to cover these newly
> introduced and future check points.
> 
> Qu Wenruo (4):
>   btrfs: relocation: Introduce error injection points for cancelling
>     balance
>   btrfs: relocation: Check cancel request after each data page read
>   btrfs: relocation: Check cancel request after each extent found
>   btrfs: relocation: Work around dead relocation stage loop
> 
>  fs/btrfs/ctree.h      |  1 +
>  fs/btrfs/relocation.c | 23 +++++++++++++++++++++++
>  fs/btrfs/volumes.c    |  2 +-
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> -- 
> 2.24.0
>