linux-kernel - Re: [DISCUSSION] proposed mctl() API

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Message-ID: <aEcTykJBgyyYYVAR@slm.duckdns.org>
Date: Mon, 9 Jun 2025 07:03:06 -1000
From: Tejun Heo <tj@...nel.org>
To: Johannes Weiner <hannes@...xchg.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, Matthew Wilcox <willy@...radead.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	David Hildenbrand <david@...hat.com>, Jann Horn <jannh@...gle.com>,
	Arnd Bergmann <arnd@...db.de>,
	Christian Brauner <brauner@...nel.org>,
	SeongJae Park <sj@...nel.org>, Usama Arif <usamaarif642@...il.com>,
	Mike Rapoport <rppt@...nel.org>, Barry Song <21cnbao@...il.com>,
	linux-mm@...ck.org, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
	Pedro Falcato <pfalcato@...e.de>
Subject: Re: [DISCUSSION] proposed mctl() API

Hello,

On Thu, Jun 05, 2025 at 08:31:56AM -0400, Johannes Weiner wrote:
> On Wed, Jun 04, 2025 at 08:19:28AM -0400, Johannes Weiner wrote:
> > On Fri, May 30, 2025 at 12:31:35PM +0200, Vlastimil Babka wrote:
...
> > > I've just read the previous threads about Barry's proposal and if doing this
> > > always isn't feasible, I'm wondering if memcg would be a better interface to
> > > opt-in for this kind of behavior than both prctl or mctl. I think at least
> > > conceptually it fits what memcg is doing? The question is if the
> > > implementation would be feasible, and if android puts apps in separate memcgs...
> > 
> > CCing Tejun.
> > 
> > Cgroups has been trying to resist flag settings like these. The cgroup
> > tree is a nested hierarchical structure designed for dividing up
> > system resources. But flag properties don't have natural inheritance
> > rules. What does it mean if the parent group says one thing and the
> > child says another? Which one has precedence?
> > 
> > Hence the proposal to make it a per-process property that propagates
> > through fork() and exec(). This also enables the container usecase (by
> > setting the flag in the container launching process), without there
> > being any confusion what the *effective* setting for any given process
> > in the system is.

+1. If something can work as something which gets inherited through the
process hierarchy, that's usually the better choice than making it a cgroup
property. There isn't much to be gained by making them cgroup properties
especially given that cgroup hierarchy, in most systems at this point, is a
degenerated process hierarchy.

Thanks.

-- 
tejun