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

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Date:   Fri, 2 Sep 2022 12:14:31 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Wei Xu <weixugc@...gle.com>, Johannes Weiner <hannes@...xchg.org>,
        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>,
        jvgediya.oss@...il.com, Bharata B Rao <bharata@....com>,
        Greg Thelen <gthelen@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH v3 updated] mm/demotion: Expose memory tier details via
 sysfs

On 9/2/22 12:10 PM, Huang, Ying wrote:
> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
> 
>> On 9/2/22 11:42 AM, Huang, Ying wrote:
>>> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
>>>
>>>> On 9/2/22 11:10 AM, Huang, Ying wrote:
>>>>> Aneesh Kumar K V <aneesh.kumar@...ux.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@...el.com> wrote:
>>>>>>>>
>>>>>>>> Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> writes:
>>>>>>>>
>>>>>>>>> On 9/1/22 12:31 PM, Huang, Ying wrote:
>>>>>>>>>> "Aneesh Kumar K.V" <aneesh.kumar@...ux.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