Re: [PATCH] memcg: add hierarchical effective limits for v2

["Michal Koutný" <mkoutny@suse.com>]

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Hello.

On Tue, Feb 11, 2025 at 05:08:03PM -0800, Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > So maybe someone can come up with a better explanation of a specific problem
> > we're trying to solve here?
 
In my experience, another factor is the switch from v1 to v2 (which
propagates slower to downstreams) and applications that rely on
memory.stat:hierarchical_memory_limit. (Funnily, enough the commit
fee7b548e6f2b ("memcg: show real limit under hierarchy mode") introduces
it primarily for debugging purposes (not sizing). An application being
killed with no apparent (immediate) limit breach.)

Roman, you may also remember that it had already popped ~year ago [1].


> The most simple explanation is visibility. Workloads that used to run
> solo are being moved to a multi-tenant but non-overcommited environment
> and they need to know their capacity which they used to get from system
> metrics.

> Now they have to get from cgroup limit files but usage of
> cgroup namespace limits those workloads to extract the needed
> information.

I remember Shakeel said the limit may be set higher in the hierarchy for
container + siblings but then it's potentially overcommitted, no?

I.e. namespace visibility alone is not the problem. The cgns root's
memory.max is the shared medium between host and guest through which the
memory allowance can be passed -- that actually sounds to me like
Johannes' option b).

(Which leads me to an idea of memory.max.effective that'd only present
the value iff there's no sibling between tightest ancestor..self. If one
looks at nr_tasks, it's partial but correct memory available. Not that
useful due to the partiality.)

Since I was originally fan of the idea, I'm not a strong opponent of
plain memory.max.effective, especially when Johannes considers the
option of kernel stepping back here and it may help some users. But I'd
like to see the original incarnations [2] somehow linked (and maybe
start only with memory.max as
that has some usecases).

Thanks,
Michal

[1] https://lore.kernel.org/all/ZcY7NmjkJMhGz8fP@host1.jankratochvil.net/
[2] https://lore.kernel.org/all/20240606152232.20253-1-mkoutny@suse.com/

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