linux-kernel - Re: [PATCH v1 00/14] mm: memcg: separate legacy cgroup v1 code and put under config option

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Date: Mon, 3 Jun 2024 12:34:57 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>
Cc: Muchun Song <muchun.song@...ux.dev>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...nel.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 00/14] mm: memcg: separate legacy cgroup v1 code and
 put under config option

Johannes, Michal,

friendly ping. Do you have any concerns regarding this patchset?

There is a number of things which can/need to be improved/polished on top,
but I'd really like to land this one first, because rebasing of
such a massive patchset is time-consuming and might also introduce
some silly bugs.

Thanks

On Tue, May 28, 2024 at 01:20:52PM -0700, Roman Gushchin wrote:
> Cgroups v2 have been around for a while and many users have fully adopted them,
> so they never use cgroups v1 features and functionality. Yet they have to "pay"
> for the cgroup v1 support anyway:
> 1) the kernel binary contains an unused cgroup v1 code,
> 2) some code paths have additional checks which are not needed,
> 3) some common structures like task_struct and mem_cgroup contain unused
>    cgroup v1-specific members.
> 
> Cgroup v1's memory controller has a number of features that are not supported
> by cgroup v2 and their implementation is pretty much self contained.
> Most notably, these features are: soft limit reclaim, oom handling in userspace,
> complicated event notification system, charge migration. Cgroup v1-specific code
> in memcontrol.c is close to 4k lines in size and it's intervened with generic
> and cgroup v2-specific code. It's a burden on developers and maintainers.
> 
> This patchset aims to solve these problems by:
> 1) moving cgroup v1-specific memcg code to the new mm/memcontrol-v1.c file,
> 2) putting definitions shared by memcontrol.c and memcontrol-v1.c into the
>    mm/memcontrol-v1.h header,
> 3) introducing the CONFIG_MEMCG_V1 config option, turned off by default,
> 4) making memcontrol-v1.c to compile only if CONFIG_MEMCG_V1 is set.
> 
> If CONFIG_MEMCG_V1 is not set, cgroup v1 memory controller is still available
> for mounting, however no memory-specific control knobs are present.
> 
> This patchset is based against mm-unstable tree (b610f75d19a34),
> however a version based on mm-stable can be found here:
>   https://github.com/rgushchin/linux/tree/memcontrol_v1.1-stable .
> 
> rfc:
>   https://lwn.net/Articles/973082/
> 
> v1:
>   - switched to CONFIG_MEMCG_V1 being off by default based on LSFMMBPF
>     discussion [1]
>   - switched to memcg1_ prefix (Johannes)
>   - many minor fixes
>   - dropped patches which put struct memcg members under CONFIG_MEMCG_V1
>     (will post as a separate patchset)
> 
> [1]: https://lwn.net/Articles/974575/