linux-kernel - Re: [RFC PATCH 0/3] Cgroup-based THP control

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Message-ID: <20241030150851.GB706616@cmpxchg.org>
Date: Wed, 30 Oct 2024 11:08:51 -0400
From: Johannes Weiner <hannes@...xchg.org>
To: gutierrez.asier@...wei-partners.com
Cc: akpm@...ux-foundation.org, david@...hat.com, ryan.roberts@....com,
	baohua@...nel.org, willy@...radead.org, peterx@...hat.com,
	hocko@...nel.org, roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
	muchun.song@...ux.dev, cgroups@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, stepanov.anatoly@...wei.com,
	alexander.kozhevnikov@...wei-partners.com, guohanjun@...wei.com,
	weiyongjun1@...wei.com, wangkefeng.wang@...wei.com,
	judy.chenhui@...wei.com, yusongping@...wei.com,
	artem.kuzin@...wei.com, kang.sun@...wei.com
Subject: Re: [RFC PATCH 0/3] Cgroup-based THP control

On Wed, Oct 30, 2024 at 04:33:08PM +0800, gutierrez.asier@...wei-partners.com wrote:
> From: Asier Gutierrez <gutierrez.asier@...wei-partners.com>
> 
> Currently THP modes are set globally. It can be an overkill if only some
> specific app/set of apps need to get benefits from THP usage. Moreover, various
> apps might need different THP settings. Here we propose a cgroup-based THP
> control mechanism.
> 
> THP interface is added to memory cgroup subsystem. Existing global THP control
> semantics is supported for backward compatibility. When THP modes are set
> globally all the changes are propagated to memory cgroups. However, when a
> particular cgroup changes its THP policy, the global THP policy in sysfs remains
> the same.
> 
> New memcg files are exposed: memory.thp_enabled and memory.thp_defrag, which
> have completely the same format as global THP enabled/defrag.
> 
> Child cgroups inherit THP settings from parent cgroup upon creation. Particular
> cgroup mode changes aren't propagated to child cgroups.

Cgroups are for hierarchical resource distribution. It's tempting to
add parameters you would want for flat collections of processes, but
it gets weird when it comes to inheritance and hiearchical semantics
inside the cgroup tree - like it does here. So this is not a good fit.

On this particular issue, I agree with what Willy and David: let's not
proliferate THP knobs; let's focus on making them truly transparent.