Re: [PATCH 0/4] btrfs: Make balance cancelling response faster

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/2/11 下午1:21, Qu Wenruo wrote:
> 
> 
> On 2019/12/5 上午12:39, David Sterba wrote:
>> 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.
> 
> When cancelled, we still merge the reloc roots with its source (if
> possible, as we still do the check for last_snapshot generation).
> 
> That means, if balance is canceled halfway, we still merge what is
> relocated. Then do the regular cleanup (cleanup the reloc tree).

My bad, that's not the case. But it doesn't matter anyway.

Since we error out by setting @err to -ECANCELD, we won't go through the
merge part.

We just mark all related reloc roots as orphan, then go through the
cleanup path without doing the merge.

That's the existing error handling code, and we should have experienced
quite some of them for the random data reloc tree csum mismatch bug.

At least the error handling part looks pretty solid.

I'll add this explanation into the cover letter of next version.

Thanks,
Qu

> 
> I see no problem doing faster canceling here.
> 
> Or do you have any extra concern?
> 
> Thanks,
> Qu
> 


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