Re: [PATCH v4 0/7] btrfs: qgroup: Delay subtree scan to reduce overhead

[Qu Wenruo <wqu@suse.de>]

[-- Attachment #1.1: Type: text/plain, Size: 4056 bytes --]



On 2019/1/16 上午9:34, Josef Bacik wrote:
> On Wed, Jan 16, 2019 at 09:29:29AM +0800, Qu Wenruo wrote:
>>
>>
>> On 2019/1/16 上午9:15, Josef Bacik wrote:
>>> On Wed, Jan 16, 2019 at 09:07:26AM +0800, Qu Wenruo wrote:
>>>>
>>>>
>>>> On 2019/1/16 上午8:31, Qu Wenruo wrote:
>>>>>
>>>>>
>>>>> On 2019/1/16 上午1:26, Josef Bacik wrote:
>>>>>> On Tue, Jan 15, 2019 at 04:15:57PM +0800, Qu Wenruo wrote:
>>>>>>> This patchset can be fetched from github:
>>>>>>> https://github.com/adam900710/linux/tree/qgroup_delayed_subtree
>>>>>>>
>>>>>>> Which is based on v5.0-rc1.
>>>>>>>
>>>>>>
>>>>>> I've been testing these patches hoping to get rid of the qgroup deadlock that
>>>>>> these patches are supposed to fix, but instead they make the box completely
>>>>>> unusable with 100% cpu usage for minutes at a time at every transaction commit.
>>>>>
>>>>> I'm afraid it's related to the v5.0-rc1 base, not the patchset itself.
>>>>>
>>>>> Just try to balance metadata with 16 snapshots, you'll see btrfs bumping
>>>>> its generation like crazy, no matter if quota is enabled or not.
>>>>>
>>>>> And since btrfs is committing transaction like crazy, no wonder it will
>>>>> do tons of qgroup accounting.
>>>>>
>>>>> My bisect leads to commit 64403612b73a94bc7b02cf8ca126e3b8ced6e921
>>>>> btrfs: rework btrfs_check_space_for_delayed_refs.
>>>>
>>>> Furthermore, you could try this RFC test case to see the problem.
>>>> https://patchwork.kernel.org/patch/10761715/
>>>>
>>>> It would only take around 100s for v4.20 but over 500 for v5.0-rc1 with
>>>> tons of unnecessary transaction committed for nothing, no quota enabled.
>>>>
>>>> So I'm afraid that commit is blocking my qgroup patchset.
>>>>
>>>
>>> I've fixed the balance problem, it took 2 seconds to figure out, I'm just
>>> waiting on xfstests to finish running.
>>>
>>> And your patch making things worse has nothing to do with that problem.  Our
>>> test doesn't run balance, so the issue you reported has nothing to do with the
>>> fact that your patch makes our boxes unusable with qgroups on.
>>>
>>> The problem is with your deadlock avoidence patch we're now making a lot more
>>> dirty extents to run in the critical section of the transaction commit.  Also
>>> because we're no longer pre-fetching the old roots, we're doing the old roots
>>> and new roots lookup inside the critical section, so now each dirty extent takes
>>> 2x as long.  With my basic test it was taking 5 minutes to do the qgroup
>>> accounting, which keeps the box from booting essentially.
>>>
>>> Without your patch it's still awful because btrfs-cleaner just sits there at
>>> 100% while deleting snapshots, but at least it's not making the whole system
>>> stop running while it does all that work in the transaction commit.
>>>
>>> And if you had done the proper root cause analysis you would have noticed that
>>> we're taking tree locks in the find_parent_nodes() case when we're searching the
>>> commit root, something we shouldn't be doing.  So all that really needs to be
>>> done is to check if (!path->skip_locking) btrfs_tree_read_lock(eb); in those
>>> cases and the deadlock goes away.  Because no matter what we shouldn't be taking
>>> locks when we're not given a trans in the backref lookup code.
>>
>> That indeed looks much better than my current solution.
>>
>> Although I'm not 100% sure for cases like tree blocks shared between
>> commit and current root (tree block not modified yet).
> 
> Doesn't matter, the block can't be modified so we don't need the locking,  If
> the current root needs to change it then it cow's it and messes with the new eb,
> not the one attached to the commit root.
> 
>>
>> I'll definitely invest more time to try to fix this bug.
>>
> 
> Don't worry about it, I'm already running the patch through my tests, I'll send
> them in the morning when the tests are finished.  Thanks,

I can't be more happier dropping that deadlock fix patch.

Thanks,
Qu

> 
> Josef
> 


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 488 bytes --]