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

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Message-ID: <ZyI0LTV2YgC4CGfW@tiehlicka>
Date: Wed, 30 Oct 2024 14:27:09 +0100
From: Michal Hocko <mhocko@...e.com>
To: Gutierrez Asier <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,
	hannes@...xchg.org, 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 30-10-24 15:51:00, Gutierrez Asier wrote:
> 
> 
> On 10/30/2024 11:38 AM, Michal Hocko wrote:
> > On Wed 30-10-24 16:33:08, 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.
> > 
> > Do you have any specific examples where this would be benefitial?
> 
> Now we're mostly focused on database scenarios (MySQL, Redis).  

That seems to be more process than workload oriented. Why the existing
per-process tuning doesn't work?

[...]
> >> Child cgroups inherit THP settings from parent cgroup upon creation. Particular
> >> cgroup mode changes aren't propagated to child cgroups.
> > 
> > So this breaks hierarchical property, doesn't it? In other words if a
> > parent cgroup would like to enforce a certain policy to all descendants
> > then this is not really possible. 
> 
> The first idea was to have some flexibility when changing THP policies. 
> 
> I will submit a new patch set which will enforce the cgroup hierarchy and change all
> the children recursively.

What is the expected semantics then?
-- 
Michal Hocko
SUSE Labs