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Date:   Wed, 8 Jun 2022 11:55:14 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc:     linux-mm@kvack.org, akpm@linux-foundation.org,
        Wei Xu <weixugc@google.com>, Huang Ying <ying.huang@intel.com>,
        Greg Thelen <gthelen@google.com>,
        Yang Shi <shy828301@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Tim C Chen <tim.c.chen@intel.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Michal Hocko <mhocko@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hesham Almatary <hesham.almatary@huawei.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Alistair Popple <apopple@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Feng Tang <feng.tang@intel.com>,
        Jagdish Gediya <jvgediya@linux.ibm.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        David Rientjes <rientjes@google.com>
Subject: Re: [PATCH v5 1/9] mm/demotion: Add support for explicit memory tiers
Message-ID: <YqDGYjgjcS5OoS3P@cmpxchg.org>
References: <20220603134237.131362-1-aneesh.kumar@linux.ibm.com>
 <20220603134237.131362-2-aneesh.kumar@linux.ibm.com>
 <YqCuE87gCcrnAiXG@cmpxchg.org>
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Hello,

On Wed, Jun 08, 2022 at 10:11:31AM -0400, Johannes Weiner wrote:
> 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

Forgive me if I'm asking a question that has been answered. I went
back to earlier threads and couldn't work it out - maybe there were
some off-list discussions? Anyway...

Why is there a distinction between tier ID and rank? I undestand that
rank was added because tier IDs were too few. But if rank determines
ordering, what is the use of a separate tier ID? IOW, why not make the
tier ID space wider and have the kernel pick a few spread out defaults
based on known hardware, with plenty of headroom to be future proof.

  $ ls tiers
  100				# DEFAULT_TIER
  $ cat tiers/100/nodelist
  0-1				# conventional numa nodes

  <pmem is onlined>

  $ grep . tiers/*/nodelist
  tiers/100/nodelist:0-1	# conventional numa
  tiers/200/nodelist:2		# pmem

  $ grep . nodes/*/tier
  nodes/0/tier:100
  nodes/1/tier:100
  nodes/2/tier:200

  <unknown device is online as node 3, defaults to 100>

  $ grep . tiers/*/nodelist
  tiers/100/nodelist:0-1,3
  tiers/200/nodelist:2

  $ echo 300 >nodes/3/tier
  $ grep . tiers/*/nodelist
  tiers/100/nodelist:0-1
  tiers/200/nodelist:2
  tiers/300/nodelist:3

  $ echo 200 >nodes/3/tier
  $ grep . tiers/*/nodelist
  tiers/100/nodelist:0-1	
  tiers/200/nodelist:2-3

etc.