linux-kernel - Re: [RFC PATCH 1/2] mm/swap, memcg: basic structure and logic for per cgroup swap priority control

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Message-ID: <aFIJDQeHmTPJrK57@yjaykim-PowerEdge-T330>
Date: Wed, 18 Jun 2025 09:32:13 +0900
From: YoungJun Park <youngjun.park@....com>
To: Michal Koutný <mkoutny@...e.com>
Cc: linux-mm@...ck.org, akpm@...ux-foundation.org, hannes@...xchg.org,
	mhocko@...nel.org, roman.gushchin@...ux.dev, shakeel.butt@...ux.dev,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com,
	bhe@...hat.com, baohua@...nel.org, chrisl@...nel.org,
	muchun.song@...ux.dev, iamjoonsoo.kim@....com, taejoon.song@....com,
	gunho.lee@....com
Subject: Re: [RFC PATCH 1/2] mm/swap, memcg: basic structure and logic for
 per cgroup swap priority control

On Tue, Jun 17, 2025 at 02:23:07PM +0200, Michal Koutný wrote:
> Hello.
> 
> On Thu, Jun 12, 2025 at 07:37:43PM +0900, youngjun.park@....com wrote:
> > Example:
> > cat memory.swap.priority
> > Inactive
> > /dev/sdb	unique:1	 prio:10
> > /dev/sdc	unique:2	 prio:5
> > 
> > - Creation
> > echo  "unique id of swapdev 1: priority, unique id of swapdev 2: priority ..."
> > > memory.swap.priority
> > 
> > - Destruction
> > Reset through the memory.swap.priority interface.
> > Example: echo "" > memory.swap.priority
> > 
> > And also be destroyed when the mem_cgroup is removed.
> > 
> > 3. Priority Mechanism
> > 
> > - Follows the original concept of swap priority.
> > (This includes automatic binding of swap devices to NUMA nodes.)
> 
> How is this supposed to work
> cg1     /dev/sda	prio:10
>         /dev/sdb	prio:5
> ` cg3     /dev/sda	  prio:5
>    	  /dev/sdb	  prio:10
> cg2     /dev/sda	prio:5
>         /dev/sdb	prio:10
> ` cg4     /dev/sda	  prio:10
>    	  /dev/sdb	  prio:5
> 
> when there are competitors from cg3 and cg4? Which device should be
> preferred by each cgroup?

Hello Michal.

What issue is the question assuming the existence of competitors in two
cgroups trying to address? Could you explain it a bit more specifically?

To answer your question for now,
Each cgroup just prefers devices according to their priority values.
until swap device is exhausted.

cg1 prefer /dev/sda than /dev/sdb.
cg2 prefer /dev/sdb than /dev/sda.
cg3 prefer /dev/sdb than /dev/sda.
cg4 prefer /dev/sda than /dev/sdb.

> Interface note -- try to make it "Nested keyed" or "Flat keyed" as
> described in Documentation/admin-guide/cgroup-v2.rst (like io.max or
> io.weight), so that it is consistent with other cgroup v2 APIs.

Yes, it looks like the API format should be adjusted as you suggested.
Thanks for the review.

Regards,
Youngjun Park