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

[Muchun Song <songmuchun@bytedance.com>]

On Sat, Sep 18, 2021 at 8:13 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Fri, Sep 17, 2021 at 06:49:21PM +0800, Muchun Song wrote:
> > On Fri, Sep 17, 2021 at 9:29 AM Roman Gushchin <guro@fb.com> 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.
>
> lruvec_memcg I mean.

Thanks for your clarification.

>
> >
> > 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?
>
> So the basic idea is simple: a lru page charged to a memcg is associated with
> a per-memcg lruvec (list_lru_memcg), which is associated with a memory cgroup.
> And after your patches there is a second link of associations: page to objcg
> to memcg:
>
> 1) page->objcg->memcg
> 2) page->list_lru_memcg->memcg
>
> (those are not necessarily direct pointers, but generally speaking, relations).
>
> My gut feeling is that if we can merge them into just 2) and use list_lru_memcg
> as an intermediate object between pages and memory cgroups, the whole thing can
> be more efficient and beautiful.
>
> Yes, on reparenting we'd need to scan over all pages in the lru list, but
> hopefully we can do it from a worker context. And it's not such a big deal as
> with slab objects, where we simple had no list of all objects.

struct list_lru_memcg seems to be redundant, it just contains a pointer
to struct mem_cgroup. We need to update each page->lruvec_memcg,
why not update page->memcg_data directly to its parent memcg?

The update of page->lruvec_memcg should be under both child and
parent's lruvec lock, right? I suppose scanning over all pages may be
a problem if there are many pages.

Thanks.

>
> Again, I'm not 100% sure if it's possible and worth it, so it shouldn't block
> your patchset if everybody else like it.
>
> Thanks