Re: [PATCH v2 0/3] mm: Make unregistration of super_block shrinker more faster

[Qi Zheng]

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@xxxxxxxxx/
> > > >
> > > > ---
> > > >
> > > > 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@xxxxxxxxxxxxxxx to this entry? So that future people
will not miss to cc fs folks.

Qi.

> -Dave.