Re: [PATCH v1 01/12] mm: memcontrol: prepare objcg API for non-kmem usage

[Muchun Song <songmuchun@bytedance.com>]

On Wed, Aug 18, 2021 at 11:01 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Sat, Aug 14, 2021 at 01:25:08PM +0800, Muchun Song wrote:
> > Pagecache pages are charged at the allocation time and holding a
> > reference to the original memory cgroup until being reclaimed.
> > Depending on the memory pressure, specific patterns of the page
> > sharing between different cgroups and the cgroup creation and
> > destruction rates, a large number of dying memory cgroups can be
> > pinned by pagecache pages. It makes the page reclaim less efficient
> > and wastes memory.
> >
> > We can convert LRU pages and most other raw memcg pins to the objcg
> > direction to fix this problem, and then the page->memcg will always
> > point to an object cgroup pointer.
> >
> > Therefore, the infrastructure of objcg no longer only serves
> > CONFIG_MEMCG_KMEM. In this patch, we move the infrastructure of the
> > objcg out of the scope of the CONFIG_MEMCG_KMEM so that the LRU pages
> > can reuse it to charge pages.
> >
> > We know that the LRU pages are not accounted at the root level. But
> > the page->memcg_data points to the root_mem_cgroup. So the
> > page->memcg_data of the LRU pages always points to a valid pointer.
> > But the root_mem_cgroup dose not have an object cgroup. If we use
> > obj_cgroup APIs to charge the LRU pages, we should set the
> > page->memcg_data to a root object cgroup. So we also allocate an
> > object cgroup for the root_mem_cgroup.
> >
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
>
> I like the "move objcg stuff to memcg/css level" part.
>
> I'm less convinced about making byte-sized charging kmem-specific
> (both naming and #ifdef CONFIG_MEMCG_KMEM). Do we really win a lot?
>
> I understand why we might wanna compile out some checks from the
> hot allocation path, but few bytes in struct objcg will not make a big
> difference, as well as few lines of code in cgroup creation/removal paths.
>
> Also it might be useful for byte-sized accounting outside kmem, e.g. zswap.
> So, I'd remove this dependency and rename to something like
> obj_cgroup_release_bytes().

Got it. I'll do that in the next version. Thanks for your suggestions.

>
> In the long run we might wanna to eliminate CONFIG_MEMCG_KMEM completely,
> so let's at least not add new dependencies.

Got it.

>
> Thanks!