Re: [RFC PATCH] mm: Add reclaim type to memory.reclaim

[Yafang Shao <laoar.shao@gmail.com>]

On Tue, Feb 27, 2024 at 5:05 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Tue 27-02-24 13:48:31, Yafang Shao wrote:
> > On Mon, Feb 26, 2024 at 10:05 PM Michal Hocko <mhocko@suse.com> wrote:
> [...]
> > > > To manage disk
> > > > storage efficiently, we employ an agent that identifies container images
> > > > eligible for destruction once all instances of that image exit.
> > > >
> > > > However, during destruction, dealing with directories containing numerous
> > > > negative dentries can significantly impact performance.
> > >
> > > Performance of what. I have to say I am kind of lost here. We are
> > > talking about memory or a disk storage?
> >
> > Removing an empty directory with numerous dentries can significantly
> > prolong the process of freeing associated dentries, leading to high
> > system CPU usage that adversely affects overall system performance.
>
> Is there anything that prevents you from reclaiming the memcg you are
> about to remove? We do have interfaces for that.

Reclaiming numerous dentries through force_empty can also lead to
potential issues, which is why we attempt to shrink the slab gradually
to mitigate them. However, it's important to note that the underlying
causes of the issues in force_empty and rmdir are not identical, as
they involve different locks.

>
> > > > To mitigate this
> > > > issue, we aim to proactively reclaim these dentries using a user agent.
> > > > Extending the memory.reclaim functionality to specifically target slabs
> > > > aligns with our requirements.
> > >
> > > Matthew has already pointed out that this has been proposed several
> > > times already and rejected.
> >
> > With that being said, we haven't come up with any superior solutions
> > compared to the proposals mentioned.
> >
> > > Dedicated slab shrinking interface is
> > > especially tricky because you would need a way to tell which shrinkers
> > > to invoke and that would be very kernel version specific.
> >
> > The persistence of this issue over several years without any
> > discernible progress suggests that we might be heading in the wrong
> > direction. Perhaps we could consider providing a kernel interface to
> > users, allowing them to tailor the reclamation process based on their
> > workload requirements.
>
> There are clear problems identified with type specific reclaim and those
> might easily strike back with future changes. Once we put an interface
> in place we won't be able remove it and that could lead to problems with
> future changes in the memory reclaim.

That shouldn't deter us from actively seeking a resolution to an issue
that has persisted for tens of years.
As observed, numerous memcg interfaces have been deprecated in recent years.

-- 
Regards
Yafang