linux-kernel - Re: [PATCH v2 0/3] mm: Make unregistration of super

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Date:   Wed, 7 Jun 2023 10:51:35 +0800
From:   Qi Zheng <qi.zheng@...ux.dev>
To:     Dave Chinner <david@...morbit.com>, Kirill Tkhai <tkhai@...ru>
Cc:     akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
        vbabka@...e.cz, viro@...iv.linux.org.uk, brauner@...nel.org,
        djwong@...nel.org, hughd@...gle.com, paulmck@...nel.org,
        muchun.song@...ux.dev, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v2 0/3] mm: Make unregistration of super_block shrinker
 more faster



On 2023/6/7 06:02, Dave Chinner wrote:
> On Wed, Jun 07, 2023 at 12:06:03AM +0300, Kirill Tkhai wrote:
>> On 06.06.2023 01:32, Dave Chinner wrote:
>>> On Mon, Jun 05, 2023 at 10:02:46PM +0300, Kirill Tkhai wrote:
>>>> This patch set introduces a new scheme of shrinker unregistration. It allows to split
>>>> the unregistration in two parts: fast and slow. This allows to hide slow part from
>>>> a user, so user-visible unregistration becomes fast.
>>>>
>>>> This fixes the -88.8% regression of stress-ng.ramfs.ops_per_sec noticed
>>>> by kernel test robot:
>>>>
>>>> https://lore.kernel.org/lkml/202305230837.db2c233f-yujie.liu@intel.com/
>>>>
>>>> ---
>>>>
>>>> Kirill Tkhai (2):
>>>>        mm: Split unregister_shrinker() in fast and slow part
>>>>        fs: Use delayed shrinker unregistration
>>>
>>> Did you test any filesystem other than ramfs?
>>>
>>> Filesystems more complex than ramfs have internal shrinkers, and so
>>> they will still be running the slow synchronize_srcu() - potentially
>>> multiple times! - in every unmount. Both XFS and ext4 have 3
>>> internal shrinker instances per mount, so they will still call
>>> synchronize_srcu() at least 3 times per unmount after this change.
>>>
>>> What about any other subsystem that runs a shrinker - do they have
>>> context depedent shrinker instances that get frequently created and
>>> destroyed? They'll need the same treatment.
>>
>> Of course, all of shrinkers should be fixed. This patch set just aims to describe
>> the idea more wider, because I'm not sure most people read replys to kernel robot reports.

Thank you, Kirill.

>>
>> This is my suggestion of way to go. Probably, Qi is right person to ask whether
>> we're going to extend this and to maintain f95bdb700bc6 in tree.
>>
>> There is not much time. Unfortunately, kernel test robot reported this significantly late.
> 
> And that's why it should be reverted rather than trying to rush to
> try to fix it.
> 
> I'm kind of tired of finding out about mm reclaim regressions only
> when I see patches making naive and/or broken changes to subsystem
> shrinker implementations without any real clue about what they are
> doing.  If people/subsystems who maintain shrinker implementations
> were cc'd on the changes to the shrinker implementation, this would
> have all been resolved before merging occurred....
> 
> Lockless shrinker lists need a heap of supporting changes to be done
> first so that they aren't reliant on synchronise_srcu() *at all*. If
> the code was properly designed in the first place (i.e. dynamic
> shrinker structures freed via call_rcu()), we wouldn't be in rushing
> to fix weird regressions right now.
> 
> Can we please revert this and start again with a properly throught
> out and reveiwed design?

I have no idea on whether to revert this, I follow the final decision of
the community.

 From my personal point of view, I think it is worth sacrificing the
speed of unregistration alone compared to the benefits it brings
(lockless shrink, etc).

Of course, it would be better if there is a more perfect solution.
If you have a better idea, it might be better to post the code first for
discussion. Otherwise, I am afraid that if we just revert it, the
problem of shrinker_rwsem will continue for many years.

And hi Dave, I know you're mad that I didn't cc you in the original
patch. Sorry again. How about splitting shrinker-related codes into
the separate files? Then we can add a MAINTAINERS entry to it and add
linux-fsdevel@...r.kernel.org to this entry? So that future people
will not miss to cc fs folks.

Qi.

> 
> -Dave.