Re: [PATCH v2 00/13] Use obj_cgroup APIs to charge the LRU pages

[Muchun Song]

On Fri, Sep 17, 2021 at 9:29 AM Roman Gushchin <guro@xxxxxx> wrote:
>
> Hi Muchun!
>
> On Thu, Sep 16, 2021 at 09:47:35PM +0800, Muchun Song wrote:
> > This version is rebased over linux 5.15-rc1, because Shakeel has asked me
> > if I could do that. I rework some code suggested by Roman as well in this
> > version. I have not removed the Acked-by tags which are from Roman, because
> > this version is not based on the folio relevant. If Roman wants me to
> > do this, please let me know, thanks.
>
> I'm fine with this, thanks for clarifying.
>
> >
> > Since the following patchsets applied. All the kernel memory are charged
> > with the new APIs of obj_cgroup.
> >
> >       [v17,00/19] The new cgroup slab memory controller[1]
> >       [v5,0/7] Use obj_cgroup APIs to charge kmem pages[2]
> >
> > But user memory allocations (LRU pages) pinning memcgs for a long time -
> > it exists at a larger scale and is causing recurring problems in the real
> > world: page cache doesn't get reclaimed for a long time, or is used by the
> > second, third, fourth, ... instance of the same job that was restarted into
> > a new cgroup every time. Unreclaimable dying cgroups pile up, waste memory,
> > and make page reclaim very inefficient.
>
> I've an idea: what if we use struct list_lru_memcg as an intermediate object
> between an individual page and struct mem_cgroup?
>
> It could contain a pointer to a memory cgroup structure (not even sure if a
> reference is needed), and a lru page can contain a pointer to the lruvec instead
> of memcg/objcg.

Hi Roman,

If I understand properly, here you mean the struct page has a pointer
to the struct lruvec not struct list_lru_memcg. What's the functionality
of the struct list_lru_memcg? Would you mind exposing more details?

>
> This approach can probably simplify the locking scheme. But what's more
> important, it can dramatically reduce the number of css_get()/put() calls.
> The latter are not particularly cheap after the deletion of a cgroup:
> they are atomic_dec()'s. As a result, the reclaim efficiency could be much
> better. The downside: we will need to update page->lruvec_memcg pointers on
> reparenting pages during the cgroup removal.

Here we need to update page->lruvec_memcg pointers one by one,
right? Because the lru lock is per lruvec, the locking scheme still need
to be as proposed by this series when the page->lruvec_memcg is
changed If I understand properly. It's likely that I don't get your point.
Looking forward to your further details.

Thanks.

>
> This is a rough idea, maybe there are significant reasons why it's not possible
> or will be way worse. But I think it's worth discussing. What do you think?
>
> Thanks!