linux-kernel - Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

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Date:   Wed, 8 Jun 2022 10:11:31 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Wei Xu <weixugc@...gle.com>, Huang Ying <ying.huang@...el.com>,
        Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers

Hi Aneesh,

On Fri, Jun 03, 2022 at 07:12:29PM +0530, Aneesh Kumar K.V wrote:
> @@ -0,0 +1,20 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_MEMORY_TIERS_H
> +#define _LINUX_MEMORY_TIERS_H
> +
> +#ifdef CONFIG_TIERED_MEMORY
> +
> +#define MEMORY_TIER_HBM_GPU	0
> +#define MEMORY_TIER_DRAM	1
> +#define MEMORY_TIER_PMEM	2
> +
> +#define MEMORY_RANK_HBM_GPU	300
> +#define MEMORY_RANK_DRAM	200
> +#define MEMORY_RANK_PMEM	100
> +
> +#define DEFAULT_MEMORY_TIER	MEMORY_TIER_DRAM
> +#define MAX_MEMORY_TIERS  3

I understand the names are somewhat arbitrary, and the tier ID space
can be expanded down the line by bumping MAX_MEMORY_TIERS.

But starting out with a packed ID space can get quite awkward for
users when new tiers - especially intermediate tiers - show up in
existing configurations. I mentioned in the other email that DRAM !=
DRAM, so new tiers seem inevitable already.

It could make sense to start with a bigger address space and spread
out the list of kernel default tiers a bit within it:

MEMORY_TIER_GPU		0
MEMORY_TIER_DRAM	10
MEMORY_TIER_PMEM	20

etc.