linux-kernel - Re: A path forward to cleaning up dying cgroups?

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Message-ID: <Z6Oq47ruBzfQh0do@google.com>
Date: Wed, 5 Feb 2025 18:16:03 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Hamza Mahfooz <hamzamahfooz@...ux.microsoft.com>, linux-mm@...ck.org,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Michal Koutný <mkoutny@...e.com>,
	Michal Hocko <mhocko@...nel.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Zach O'Keefe <zokeefe@...gle.com>, Kinsey Ho <kinseyho@...gle.com>,
	Yosry Ahmed <yosryahmed@...gle.com>,
	Allen Pais <apais@...ux.microsoft.com>
Subject: Re: A path forward to cleaning up dying cgroups?

On Wed, Feb 05, 2025 at 01:08:42PM -0500, Johannes Weiner wrote:
> On Wed, Feb 05, 2025 at 12:50:19PM -0500, Hamza Mahfooz wrote:
> > Cc: Shakeel Butt <shakeel.butt@...ux.dev>
> > 
> > On 2/5/25 12:48, Hamza Mahfooz wrote:
> > > I was just curious as to what the status of the issue described in [1]
> > > is. It appears that the last time someone took a stab at it was in [2].
> 
> If memory serves, the sticking point was whether pages should indeed
> be reparented on cgroup death, or whether they could be moved
> arbitrarily to other cgroups that are still using them.
> 
> It's a bit unfortunate, because the reparenting patches were tested
> and reviewed, and the arbitrary recharging was just an idea that
> ttbomk nobody seriously followed up on afterwards.

There was an RFC series [1] for the recharging, but all memcg
maintainers hated it :P

https://lore.kernel.org/lkml/20230720070825.992023-1-yosryahmed@google.com/

> 
> We also recently removed the charge moving code from cgroup1, along
> with the subtle page access/locking/accounting rules it imposed on the
> rest of the MM. I'm doubtful there is much appetite in either camp for
> bringing this back.

Yeah with the charge moving code gone the case for recharging grows
weaker.

> 
> So I would still love to see Muchun's patches merged. They fix a
> seemingly universally experienced operational issue in memcg, and we
> shouldn't hold it up unless somebody actually posts alternative code.
> 
> Thoughts?

Adding Zach and Kinsey who were recently looking into this from the
Google side.