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Date:   Wed, 4 Dec 2019 17:39:54 +0100
From:   David Sterba <dsterba@suse.cz>
To:     Qu Wenruo <wqu@suse.com>
Cc:     linux-btrfs@vger.kernel.org
Subject: Re: [PATCH 0/4] btrfs: Make balance cancelling response faster
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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.

Yes that's the reason why it takes so long to cancel. Adding more
cancellation points is fine, but I don't know what exactly happens when
the block group relocation is not finished. There's code to merge the
reloc inode and commit that, but that's only a high-level view of the
thing.