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SubjectRe: [RFC PATCH 1/2] mm/demotion: Expose memory type details via sysfs
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On 8/26/22 7:20 AM, Huang, Ying wrote:
> "Aneesh Kumar K.V" <aneesh.kumar@linux.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

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