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Date: Thu, 9 May 2024 10:22:05 -0400
From: Johannes Weiner <hannes@cmpxchg.org>
To: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Muchun Song <muchun.song@linux.dev>,
	Michal Hocko <mhocko@kernel.org>,
	Shakeel Butt <shakeel.butt@linux.dev>,
	Matthew Wilcox <willy@infradead.org>, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH rfc 0/9] mm: memcg: separate legacy cgroup v1 code and
 put under config option
Message-ID: <20240509142205.GC374370@cmpxchg.org>
References: <20240509034138.2207186-1-roman.gushchin@linux.dev>
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In-Reply-To: <20240509034138.2207186-1-roman.gushchin@linux.dev>

On Wed, May 08, 2024 at 08:41:29PM -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 useless cgroup v1 code,
> 2) some common structures like task_struct and mem_cgroup have never used
>    cgroup v1-specific members,
> 3) some code paths have additional checks which are not needed.
> 
> 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.

Great patchset. The moves look clean and straight-forward to me on
first glance.

> This patchset aims to solve these problems by:
> 1) moving cgroup v1-specific memcg code to the new mm/memcontrol-v1.c file,

+1

> 2) putting definitions shared by memcontrol.c and memcontrol-v1.c into the
>    mm/internal.h header

You proposed mm/memcontrol-v1.h below, IMO that's the best option.

> 3) introducing the CONFIG_MEMCG_V1 config option, turned on by default

+1

CONFIG_MEMCG1 should also work.

> 4) making memcontrol-v1.c to compile only if CONFIG_MEMCG_V1 is set

+1

> 5) putting unused struct memory_cgroup and task_struct members under
>    CONFIG_MEMCG_V1 as well.

+1

> 
> This is an RFC version, which is not 100% polished yet, so but it would be great
> to discuss and agree on the overall approach.
> 
> Some open questions, opinions are appreciated:
> 1) I consider renaming non-static functions in memcontrol-v1.c to have
>    mem_cgroup_v1_ prefix. Is this a good idea?

I think this would be great, to make it more obvious in memcontrol.c.

For core cgroup code, we used cgroup1_foo(). Maybe name them all
things like memcg1_update_tree() etc.? That's short and sweet while
sticking out visually pretty well.

> 2) Do we want to extend it beyond the memory controller? Should

Could you please elaborate? ^_^

> 3) Is it better to use a new include/linux/memcontrol-v1.h instead of
>    mm/internal.h? Or mm/memcontrol-v1.h.

mm/memcontrol-v1.h sounds good to me.

>  mm/memcontrol.c            | 4121 ++++++++++++++++++++++---------------------------------------------------------------------------------------------------------------------------------

Lol, awesome.