linux-kernel - Re: [RFC PATCH 1/2] mm/demotion: Expose memory type details via sysfs

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Message-ID: <4abb84e8-8035-65aa-941a-98f0d7902c42@linux.ibm.com>
Date:   Fri, 26 Aug 2022 13:35:44 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Wei Xu <weixugc@...gle.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.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>,
        Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com,
        Bharata B Rao <bharata@....com>
Subject: Re: [RFC PATCH 1/2] mm/demotion: Expose memory type details via sysfs

On 8/26/22 1:30 PM, Wei Xu wrote:
> On Thu, Aug 25, 2022 at 8:00 PM Aneesh Kumar K V
> <aneesh.kumar@...ux.ibm.com> wrote:
>>
>> On 8/26/22 7:20 AM, Huang, Ying wrote:
>>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com> writes:
>>>
>>>> This patch adds /sys/devices/virtual/memtier/ where all memory tier related
>>>> details can be found. All allocated memory types will be listed there as
>>>> /sys/devices/virtual/memtier/memtypeN/
>>>
>>> Another choice is to make memory types and memory tiers system devices.
>>> That is,
>>>
>>> /sys/devices/system/memory_type/memory_typeN
>>> /sys/devices/system/memory_tier/memory_tierN
>>>
>>
>> subsys_system_register() documentation says
>>
>>  * Do not use this interface for anything new, it exists for compatibility
>>  * with bad ideas only. New subsystems should use plain subsystems; and
>>  * add the subsystem-wide attributes should be added to the subsystem
>>  * directory itself and not some create fake root-device placed in
>>  * /sys/devices/system/<name>.
>>
>> memtier being a virtual device, I was under the impression that /sys/devices/virtual
>> is the recommended place.
>>
>>> That looks more natural to me.  Because we already have "node" and
>>> "memory" devices there.  Why don't you put memory types and memory tiers
>>> there?
>>>
>>> And, I think we shouldn't put "memory_type" in the "memory_tier"
>>> directory.  "memory_type" isn't a part of "memory_tier".
>>>
>>
>> I was looking consolidating both memory tier and memory type into the same sysfs subsystem.
>> Your recommendation imply we create two subsystem memory_tier and memtype. I was
>> trying to avoid that. May be a generic term like "memory_tiering" can help to
>> consolidate all tiering related details there?
>>
> 
> A generic term "memory_tiering" sounds good to me.
> 
> Given that this will be a user-facing, stable kernel API, I think we'd
> better to only add what is most useful for userspace and don't have to
> mirror the kernel internal data structures in this interface.
> 
> My understanding is that we haven't fully settled down on how to
> customize memory tiers from userspace.  So we don't have to show
> memory_type yet, which is a kernel data structure at this point.
> 
> The userspace does need to know what are the memory tiers and which
> NUMA nodes are included in each memory tier.  How about we provide the
> "nodelist" interface for each memory tier as in the original proposal?
> 
> The userspace would also like to know which memory tiers/nodes belong
> to the top tiers (the promotion targets).  We can provide a "toptiers"
> or "toptiers_nodelist" interface to report that.
> 

How about also including abstract distance range of a memory tier?
That will be useful to derive the hierarchy.

> Both should still be useful even if we decide to add memory_type for
> memory tier customization.
> 

-aneesh