Re: [PATCH rfc 0/9] mm: memcg: separate legacy cgroup v1 code and put under config option

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, May 08, 2024 at 11:33:07PM -0700, Shakeel Butt wrote:
> 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.
> > 
> > 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/internal.h header
> > 3) introducing the CONFIG_MEMCG_V1 config option, turned on by default
> > 4) making memcontrol-v1.c to compile only if CONFIG_MEMCG_V1 is set
> > 5) putting unused struct memory_cgroup and task_struct members under
> >    CONFIG_MEMCG_V1 as well.
> > 
> > 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?
> > 2) Do we want to extend it beyond the memory controller? Should
> > 3) Is it better to use a new include/linux/memcontrol-v1.h instead of
> >    mm/internal.h? Or mm/memcontrol-v1.h.
> > 
> 
> Hi Roman,
> 
> A very timely and important topic and we should definitely talk about it
> during LSFMM as well. I have been thinking about this problem for quite
> sometime and I am getting more and more convinced that we should aim to
> completely deprecate memcg-v1.
> 
> More specifically:
> 
> 1. What are the memcg-v1 features which have no alternative in memcg-v2
> and are blocker for memcg-v1 users? (setting aside the cgroup v2
> structual restrictions)

I don't think there are any, except there might be a certain cost to migrate,
so some companies might be resistant to put in resources, because they don't
see any immediate benefits as well.

> 
> 2. What are unused memcg-v1 features which we should start deprecating?
> 
> IMO we should systematically start deprecating memcg-v1 features and
> start unblocking the users stuck on memcg-v1.

I'm not sure we want to deprecate them one-by-one - it's a lot of work
and maybe we can deprecate it all together instead.

I think the only feature which we might want to deprecate separately -
it's the charge migration. It's the most annoying feature as it requires
a lot more synchronization, which can be dropped otherwise, so it's
complicating a lot of things. Other features are more or less self-contained.

> 
> Now regarding the proposal in this series, I think it can be a first
> step but should not give an impression that we are done.

Yeah, it's really only a first step.

> The only
> concern I have is the potential of "out of sight, out of mind" situation
> with this change but if we keep the momentum of deprecation of memcg-v1
> it should be fine.

My rough plan here:
1) move it out to a separate file and put under a config option, default on
2) clean up all remaining small bits here and there
... < wait a year >
3) flip the config option to be off by default
... < wait another year or two >
4) drop the code entirely

> 
> I have CCed Greg and David from Google to get their opinion on what
> memcg-v1 features are blocker for their memcg-v2 migration and if they
> have concern in deprecation of memcg-v1 features.

Thank you!