linux-kernel - Re: [RFC PATCH v5 0/3] Add memory.max.effective for application's allocators

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Message-ID: <ZmH8pNkk2MHvvCzb@P9FQF9L96D>
Date: Thu, 6 Jun 2024 11:15:00 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Michal Koutný <mkoutny@...e.com>
Cc: cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Jonathan Corbet <corbet@....net>, Michal Hocko <mhocko@...nel.org>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Jan Kratochvil (Azul)" <jkratochvil@...l.com>
Subject: Re: [RFC PATCH v5 0/3] Add memory.max.effective for application's
 allocators

On Thu, Jun 06, 2024 at 05:22:29PM +0200, Michal Koutný wrote:
> Some applications use memory cgroup limits to scale their own memory
> needs. Reading of the immediate membership cgroup's memory.max is not
> sufficient because of possible ancestral limits. The application could
> traverse upwards to figure out the tightest limit but this would not
> work in cgroup namespace where the view of cgroup hierarchy is
> incomplete and the limit may apply from outer world.
> Additionally, applications should respond to limit changes.

If the goal is to detect how much memory would it be possible to allocate,
I'm not sure that knowing all memory.max limits upper in the hierarchy
really buys anything without knowing actual usages and a potential
for memory reclaim across the entire tree.

E.g.:

A (max = 100G)
| \
B  C

C's effective max will come out as 100G, but if B.anon_usage = 100G and
there is no swap, the actual number is 0.

But if it's more about exploring the "invisible" part of the cgroup
tree configuration, it makes sense to me.
Not sure about the naming, maybe something like memory.tree.max
or memory.parent.max or even memory.hierarchical.max is a better fit.

Thanks!