linux-kernel - Re: [RFC PATCH] mm, memcg: introduce memory.high.throttle

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Message-ID: <Z5s1DG2YVH78RWpR@tiehlicka>
Date: Thu, 30 Jan 2025 09:15:08 +0100
From: Michal Hocko <mhocko@...e.com>
To: Waiman Long <longman@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Jonathan Corbet <corbet@....net>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-doc@...r.kernel.org,
	Peter Hunt <pehunt@...hat.com>
Subject: Re: [RFC PATCH] mm, memcg: introduce memory.high.throttle

On Wed 29-01-25 14:12:04, Waiman Long wrote:
> Since commit 0e4b01df8659 ("mm, memcg: throttle allocators when failing
> reclaim over memory.high"), the amount of allocator throttling had
> increased substantially. As a result, it could be difficult for a
> misbehaving application that consumes increasing amount of memory from
> being OOM-killed if memory.high is set. Instead, the application may
> just be crawling along holding close to the allowed memory.high memory
> for the current memory cgroup for a very long time especially those
> that do a lot of memcg charging and uncharging operations.
> 
> This behavior makes the upstream Kubernetes community hesitate to
> use memory.high. Instead, they use only memory.max for memory control
> similar to what is being done for cgroup v1 [1].

Why is this a problem for them?

> To allow better control of the amount of throttling and hence the
> speed that a misbehving task can be OOM killed, a new single-value
> memory.high.throttle control file is now added. The allowable range
> is 0-32.  By default, it has a value of 0 which means maximum throttling
> like before. Any non-zero positive value represents the corresponding
> power of 2 reduction of throttling and makes OOM kills easier to happen.

I do not like the interface to be honest. It exposes an implementation
detail and casts it into a user API. If we ever need to change the way
how the throttling is implemented this will stand in the way because
there will be applications depending on a behavior they were carefuly
tuned to.

It is also not entirely sure how is this supposed to be used in
practice? How do people what kind of value they should use?

> System administrators can now use this parameter to determine how easy
> they want OOM kills to happen for applications that tend to consume
> a lot of memory without the need to run a special userspace memory
> management tool to monitor memory consumption when memory.high is set.

Why cannot they achieve the same with the existing events/metrics we
already do provide? Most notably PSI which is properly accounted when
a task is throttled due to memory.high throttling.
-- 
Michal Hocko
SUSE Labs