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

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Message-ID: <YUPvVYFyBR/qwy2X@carbon.DHCP.thefacebook.com>
Date:   Thu, 16 Sep 2021 18:28:53 -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>, <alexs@...nel.org>, <smuchun@...il.com>,
        <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v2 00/13] Use obj_cgroup APIs to charge the LRU pages

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.

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.

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!