linux-kernel - Re: [RFC PATCH 2/2] mm: swap: apply per cgroup swap priority mechansim on swap layer

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Message-ID: <CAKEwX=PsGKS5JHqQ-G29Fg8xLssPhM+E-4wV_QakhqrDOsV36g@mail.gmail.com>
Date: Thu, 12 Jun 2025 10:28:33 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: youngjun.park@....com, 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, 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 2/2] mm: swap: apply per cgroup swap priority
 mechansim on swap layer

On Thu, Jun 12, 2025 at 4:14 AM Kairui Song <ryncsn@...il.com> wrote:
>
> On Thu, Jun 12, 2025 at 6:43 PM <youngjun.park@....com> wrote:
> >
> > From: "youngjun.park" <youngjun.park@....com>
> >
>
> Hi, Youngjun,
>
> Thanks for sharing this series.
>
> > This patch implements swap device selection and swap on/off propagation
> > when a cgroup-specific swap priority is set.
> >
> > There is one workaround to this implementation as follows.
> > Current per-cpu swap cluster enforces swap device selection based solely
> > on CPU locality, overriding the swap cgroup's configured priorities.
>
> I've been thinking about this, we can switch to a per-cgroup-per-cpu
> next cluster selector, the problem with current code is that swap

What about per-cpu-per-order-per-swap-device :-? Number of swap
devices is gonna be smaller than number of cgroups, right?

At swap slot allocation time, we check the folio's swap device
priority list, then pump that all the way to the swap allocator.

swap allocator, given a priority list, for each priority level, try to
allocate from that level first. It will get a cluster (either locally
cached or a new one) from swap devices in that priority level, before
moving on to the next priority level.