linux-kernel - Re: [RFC PATCH v3 01/12] mm: memcontrol: move the objcg infrastructure out of CONFIG_MEMCG

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Message-ID: <YK0lckLSLxhZMk3S@carbon.dhcp.thefacebook.com>
Date:   Tue, 25 May 2021 09:27:30 -0700
From:   Roman Gushchin <guro@...com>
To:     Muchun Song <songmuchun@...edance.com>
CC:     <hannes@...xchg.org>, <mhocko@...nel.org>,
        <akpm@...ux-foundation.org>, <shakeelb@...gle.com>,
        <vdavydov.dev@...il.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <duanxiongchun@...edance.com>,
        <fam.zheng@...edance.com>, <bsingharora@...il.com>,
        <shy828301@...il.com>, <alex.shi@...ux.alibaba.com>
Subject: Re: [RFC PATCH v3 01/12] mm: memcontrol: move the objcg
 infrastructure out of CONFIG_MEMCG_KMEM

On Wed, Apr 21, 2021 at 03:00:48PM +0800, Muchun Song wrote:
> Because memory allocations 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.
> 
> 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.

Overall the patch looks very good to me. There are few small things to enhance:

1) I'd rename it. Looking at the title I expect a trivial code move,
   however the patch is doing more than this: e.g. allocating an objcg
   for the root memcg. Something like "prepare objcg API for non-kmem usage".
2) How about obj_cgroup_release_kmem() instead of obj_cgroup_release_uncharge()?
3) The first paragraph of the commit log looks a bit vague: which allocations
   pinning memcgs? How about something like this?

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.

Thanks!