linux-kernel - Re: [PATCH 1/2] mm: memory-tiers, numa_emu: enable to create memory tiers using fake numa nodes

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Message-ID: <CAC5umyjCXSm1TMiv+OOdK1vHp5nA7zGRxLegMzhbLd9U=3Dbhg@mail.gmail.com>
Date: Wed, 17 Dec 2025 22:55:16 +0900
From: Akinobu Mita <akinobu.mita@...il.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, axelrasmussen@...gle.com, yuanchu@...gle.com, 
	weixugc@...gle.com, hannes@...xchg.org, david@...nel.org, mhocko@...nel.org, 
	zhengqi.arch@...edance.com, shakeel.butt@...ux.dev, 
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com, vbabka@...e.cz, 
	rppt@...nel.org, surenb@...gle.com
Subject: Re: [PATCH 1/2] mm: memory-tiers, numa_emu: enable to create memory
 tiers using fake numa nodes

2025年12月17日(水) 5:24 Andrew Morton <akpm@...ux-foundation.org>:
>
> On Mon,  8 Dec 2025 18:40:27 +0900 Akinobu Mita <akinobu.mita@...il.com> wrote:
>
> > This makes it possible to create memory tiers using fake numa nodes
> > generated by numa emulation.
> >
> > The new "numa_emulation.adistance" kernel parameter allows you to set the
> > abstract distance for each NUMA node.
> >
> > For example, if the system is booted with the parameters
> > "numa=fake=2 numa_emulation.adistance=576,704", it will configure memory
> > tiers with node0 having the default DRAM adistance value and node1 having
> > a lower adistance value.
>
> Confusing.  I'd have thought that this commandline would gave node0 a
> distance of 576 and node1 a distance of 704?  But the text talks about
> some third "default" distance, of unknown value.
>
> Can we please clear all this up?

The DRAM abstract distance is defined by MEMTIER_ADISTANCE_DRAM
in linux/memory-tiers.h and has a value of 576.

Each memory tier covers an abstract distance chunk size of 128,
so nodes with abstract distances between 512 and 639 are classified
into the DRAM tier.

Here, the abstract distances of node0 and node1 are set to 576 and 706,
respectively, so they are classified into different tiers.

> Also, we have little documentation for this stuff.
> fake-numa-for-cpusets.rst and kernel-parameters.txt.  Can you please
> find somewhere appropriate to document this new user-facing feature?
> Maybe a new Documentation file?

Looks good.
I'll create a new Documentation/mm/numa_emulation.rst and
document at least this new parameter.