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

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Message-ID: <20250605123156.GA2812@cmpxchg.org>
Date: Thu, 5 Jun 2025 08:31:56 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: 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>, Tejun Heo <tj@...nel.org>
Subject: Re: [DISCUSSION] proposed mctl() API

CCing Tejun - with the right mutt alias this time.

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:
> > On 5/29/25 23:14, Johannes Weiner wrote:
> > > On Thu, May 29, 2025 at 04:28:46PM +0100, Matthew Wilcox wrote:
> > >> Barry's problem is that we're all nervous about possibly regressing
> > >> performance on some unknown workloads.  Just try Barry's proposal, see
> > >> if anyone actually compains or if we're just afraid of our own shadows.
> > > 
> > > I actually explained why I think this is a terrible idea. But okay, I
> > > tried the patch anyway.
> > > 
> > > This is 'git log' on a hot kernel repo after a large IO stream:
> > > 
> > >                                      VANILLA                      BARRY
> > > Real time                 49.93 (    +0.00%)         60.36 (   +20.48%)
> > > User time                 32.10 (    +0.00%)         32.09 (    -0.04%)
> > > System time               14.41 (    +0.00%)         14.64 (    +1.50%)
> > > pgmajfault              9227.00 (    +0.00%)      18390.00 (   +99.30%)
> > > workingset_refault_file  184.00 (    +0.00%)    236899.00 (+127954.05%)
> > > 
> > > Clearly we can't generally ignore page cache hits just because the
> > > mmaps() are intermittent.
> > > 
> > > The whole point is to cache across processes and their various
> > > apertures into a common, long-lived filesystem space.
> > > 
> > > Barry knows something about the relationship between certain processes
> > > and certain files that he could exploit with MADV_COLD-on-exit
> > > semantics. But that's not something the kernel can safely assume. Not
> > > without defeating the page cache for an entire class of file accesses.
> > 
> > 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.