Re: [PATCH v3 updated] mm/demotion: Expose memory tier details via sysfs

Aneesh Kumar K V
2022-09-02T09:45:06Z

On 9/2/22 2:34 PM, Huang, Ying wrote:
> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:
>
>> On 9/2/22 1:27 PM, Huang, Ying wrote:
>>> Wei Xu <weixugc@google.com> writes:
>>>
>>>> On Thu, Sep 1, 2022 at 11:44 PM Aneesh Kumar K V
>>>> <aneesh.kumar@linux.ibm.com> wrote:
>>>>>
>>>>> On 9/2/22 12:10 PM, Huang, Ying wrote:
>>>>>> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:
>>>>>>
>>>>>>> On 9/2/22 11:42 AM, Huang, Ying wrote:
>>>>>>>> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:
>>>>>>>>
>>>>>>>>> On 9/2/22 11:10 AM, Huang, Ying wrote:
>>>>>>>>>> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:
>>>>>>>>>>
>>>>>>>>>>> On 9/2/22 10:39 AM, Wei Xu wrote:
>>>>>>>>>>>> On Thu, Sep 1, 2022 at 5:33 PM Huang, Ying <ying.huang@intel.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> Aneesh Kumar K V <aneesh.kumar@linux.ibm.com> writes:
>>>>>>>>>>>>>
>>>>>>>>>>>>>> On 9/1/22 12:31 PM, Huang, Ying wrote:
>>>>>>>>>>>>>>> "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com> writes:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> This patch adds /sys/devices/virtual/memory_tiering/ where all memory tier
>>>>>>>>>>>>>>>> related details can be found. All allocated memory tiers will be listed
>>>>>>>>>>>>>>>> there as /sys/devices/virtual/memory_tiering/memory_tierN/
>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>> The nodes which are part of a specific memory tier can be listed via
>>>>>>>>>>>>>>>> /sys/devices/virtual/memory_tiering/memory_tierN/nodes
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> I think "memory_tier" is a better subsystem/bus name than
>>>>>>>>>>>>>>> memory_tiering. Because we have a set of memory_tierN devices inside.
>>>>>>>>>>>>>>> "memory_tier" sounds more natural. I know this is subjective, just my
>>>>>>>>>>>>>>> preference.
>>>>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> I missed replying to this earlier. I will keep memory_tiering as subsystem name in v4
>>>>>>>>>>> because we would want it to a susbsystem where all memory tiering related details can be found
>>>>>>>>>>> including memory type in the future. This is as per discussion
>>>>>>>>>>>
>>>>>>>>>>> https://lore.kernel.org/linux-mm/CAAPL-u9TKbHGztAF=r-io3gkX7gorUunS2UfstudCWuihrA=0g@mail.gmail.com
>>>>>>>>>>
>>>>>>>>>> I don't think that it's a good idea to mix 2 types of devices in one
>>>>>>>>>> subsystem (bus). If my understanding were correct, that breaks the
>>>>>>>>>> driver core convention.
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>> All these are virtual devices .I am not sure i follow what you mean by 2 types of devices.
>>>>>>>>> memory_tiering is a subsystem that represents all the details w.r.t memory tiering. It shows
>>>>>>>>> details of memory tiers and can possibly contain details of different memory types .
>>>>>>>>
>>>>>>>> IMHO, memory_tier and memory_type are 2 kind of devices. They have
>>>>>>>> almost totally different attributes (sysfs file). So, we should create
>>>>>>>> 2 buses for them. Each has its own attribute group. "virtual" itself
>>>>>>>> isn't a subsystem.
>>>>>>>
>>>>>>> Considering both the details are related to memory tiering, wouldn't it be much simpler we consolidate
>>>>>>> them within the same subdirectory? I am still not clear why you are suggesting they need to be in different
>>>>>>> sysfs hierarchy. It doesn't break any driver core convention as you mentioned earlier.
>>>>>>>
>>>>>>> /sys/devices/virtual/memory_tiering/memory_tierN
>>>>>>> /sys/devices/virtual/memory_tiering/memory_typeN
>>>>>>
>>>>>> I think we should add
>>>>>>
>>>>>> /sys/devices/virtual/memory_tier/memory_tierN
>>>>>> /sys/devices/virtual/memory_type/memory_typeN
>>>>>>
>>>>>
>>>>> I am trying to find if there is a technical reason to do the same?
>>>>>
>>>>>> I don't think this is complex. Devices of same bus/subsystem should
>>>>>> have mostly same attributes. This is my understanding of driver core
>>>>>> convention.
>>>>>>
>>>>>
>>>>> I was not looking at this from code complexity point. Instead of having multiple directories
>>>>> with details w.r.t memory tiering, I was looking at consolidating the details
>>>>> within the directory /sys/devices/virtual/memory_tiering. (similar to all virtual devices
>>>>> are consolidated within /sys/devics/virtual/).
>>>>>
>>>>> -aneesh
>>>>
>>>> Here is an example of /sys/bus/nd/devices (I know it is not under
>>>> /sys/devices/virtual, but it can still serve as a reference):
>>>>
>>>> ls -1 /sys/bus/nd/devices
>>>>
>>>> namespace2.0
>>>> namespace3.0
>>>> ndbus0
>>>> nmem0
>>>> nmem1
>>>> region0
>>>> region1
>>>> region2
>>>> region3
>>>>
>>>> So I think it is not unreasonable if we want to group memory tiering
>>>> related interfaces within a single top directory.
>>>
>>> Thanks for pointing this out. My original understanding of driver core
>>> isn't correct.
>>>
>>> But I still think it's better to separate instead of mixing memory_tier
>>> and memory_type. Per my understanding, memory_type shows information
>>> (abstract distance, latency, bandwidth, etc.) of memory types (and
>>> nodes), it can be useful even without memory tiers. That is, memory
>>> types describes the physical characteristics, while memory tier reflects
>>> the policy.
>>>
>>
>> The latency and bandwidth details are already exposed via
>>
>> /sys/devices/system/node/nodeY/access0/initiators/
>>
>> Documentation/admin-guide/mm/numaperf.rst
>>
>> That is the interface that libraries like libmemkind will look at for finding
>> details w.r.t latency/bandwidth
>
> Yes. Only with that, it's still inconvenient to find out which nodes
> belong to same memory type (has same performance, same topology, managed
> by same driver, etc). So memory types can still provide useful
> information even without memory tiering.
>

I am not sure i quiet follow what to conclude from your reply. I used the subsystem name
"memory_tiering" so that all memory tiering related information can be consolidated there.
I guess you agreed to the above part that we can consolidated things like that.

We might end up adding memory_type there if we allow changing "abstract distance" of a
memory type from userspace later. Otherwise, I don't see a reason for memory type to be
exposed. But then we don't have to decide on this now.

-aneesh