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

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Message-ID: <e2f20ae9-5761-c170-a4e7-121d6b69ebfb@linux.ibm.com>
Date:   Fri, 26 Aug 2022 08:07:28 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Wei Xu <weixugc@...gle.com>, 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 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? 


>> The nodes which are part of a specific memory type can be listed via
>> /sys/devices/system/memtier/memtypeN/nodes.
> 
> How about create links to /sys/devices/system/node/nodeN in
> "memory_type".  But I'm OK to have "nodes" file too.
> 
>> The adistance value of a specific memory type can be listed via
>> /sys/devices/system/memtier/memtypeN/adistance.
>>
>> A directory listing looks like:
>> :/sys/devices/virtual/memtier# tree memtype1
>> memtype1
>> ├── adistance
> 
> Why not just use "abstract_distance"?  This is user space interface,
> it's better to be intuitive.
> 
>> ├── nodes
>> ├── subsystem -> ../../../../bus/memtier
>> └── uevent
>>
>> Since we will be using struct device to expose details via sysfs, drop struct
>> kref and use struct device for refcounting the memtype.
>>
> 
> Best Regards,
> Huang, Ying