Date:   Tue, 30 Aug 2022 12:58:09 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Wei Xu <weixugc@...gle.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.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: [PATCH v2] mm/demotion: Expose memory tier details via sysfs

On 8/30/22 12:47 PM, Wei Xu wrote:
> On Mon, Aug 29, 2022 at 11:46 PM Aneesh Kumar K V
> <aneesh.kumar@...ux.ibm.com> wrote:
>>
>> On 8/30/22 12:01 PM, Wei Xu wrote:
>>> On Sun, Aug 28, 2022 at 11:08 PM Aneesh Kumar K.V
>>> <aneesh.kumar@...ux.ibm.com> wrote:
>>>>
>>>> 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
>>>>
>>>> The abstract distance range value of a specific memory tier can be listed via
>>>> /sys/devices/virtual/memory_tiering/memory_tierN/abstract_distance
>>>>
>>>> A directory hierarchy looks like
>>>> :/sys/devices/virtual/memory_tiering$ tree memory_tier4/
>>>> memory_tier4/
>>>> ├── abstract_distance
>>>> ├── nodes
>>>> ├── subsystem -> ../../../../bus/memory_tiering
>>>> └── uevent
>>>>
>>>> All toptier nodes are listed via
>>>> /sys/devices/virtual/memory_tiering/toptier_nodes
>>>>
>>>> :/sys/devices/virtual/memory_tiering$ cat toptier_nodes
>>>> 0,2
>>>> :/sys/devices/virtual/memory_tiering$ cat memory_tier4/nodes
>>>> 0,2
>>>> :/sys/devices/virtual/memory_tiering$ cat memory_tier4/abstract_distance
>>>> 512 - 639
>>>>
>>>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
>>>> ---
>>>>  .../ABI/testing/sysfs-kernel-mm-memory-tiers  |  41 +++++
>>>>  mm/memory-tiers.c                             | 155 +++++++++++++++---
>>>>  2 files changed, 174 insertions(+), 22 deletions(-)
>>>>  create mode 100644 Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers
>>>>
>>>> diff --git a/Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers b/Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers
>>>> new file mode 100644
>>>> index 000000000000..6955f69a4423
>>>> --- /dev/null
>>>> +++ b/Documentation/ABI/testing/sysfs-kernel-mm-memory-tiers
>>>> @@ -0,0 +1,41 @@
>>>> +What:          /sys/devices/virtual/memory_tiering/
>>>> +Date:          August 2022
>>>> +Contact:       Linux memory management mailing list <linux-mm@...ck.org>
>>>> +Description:   A collection of all the memory tiers allocated.
>>>> +
>>>> +               Individual memory tier details are contained in subdirectories
>>>> +               named by the abstract distance of the memory tier.
>>>> +
>>>> +               /sys/devices/virtual/memory_tiering/memory_tierN/
>>>> +
>>>> +
>>>> +What:          /sys/devices/virtual/memory_tiering/memory_tierN/
>>>> +               /sys/devices/virtual/memory_tiering/memory_tierN/abstract_distance
>>>> +               /sys/devices/virtual/memory_tiering/memory_tierN/nodes
>>>> +Date:          August 2022
>>>> +Contact:       Linux memory management mailing list <linux-mm@...ck.org>
>>>> +Description:   Directory with details of a specific memory tier
>>>> +
>>>> +               This is the directory containing information about a particular
>>>> +               memory tier, memtierN, where N is derived based on abstract distance.
>>>> +
>>>> +               A smaller value of N implies a higher (faster) memory tier in the
>>>> +               hierarchy.
>>>
>>> Given that abstract_distance is provided, it would be more flexible if
>>> we don't commit to the interface where N in memtierN also indicates
>>> the memory tier ordering.
>>
>>
>> IIUC this is one of the request that Johannes had ie, to be able to understand the
>> memory tier hierarchy based on memtier name.
>>
>>>> +
>>>> +               abstract_distance: The abstract distance range this specific memory
>>>> +               tier maps to.
>>>
>>> I still think the name of "abstract distance" is kind of confusing
>>> because it is not clear what is the other object that this distance
>>> value is relative to.  Do we have to expose this value at this point
>>> if N in memtierN can already indicate the memory tier ordering?
>>>
>>
>> I do agree that abstract distance is confusing. But IIUC we agreed that it is much better
>> than other names suggested and is closer to already understood "numa distance" term.
>>
>> https://lore.kernel.org/linux-mm/YuLF%2FGG8x5lQvg%2Ff@cmpxchg.org/
>>
> 
> "NUMA distance" measures the distance between two NUMA nodes.
> 
> I bring it up again because this name will become a user visible
> kernel interface, which we will need to live with for a long time.
> Even if we decide to keep the name, it would be better if we can
> define between which two (abstract) points the abstract distance
> reports.
> 
> Another option is to remove this interface for now until it becomes
> necessary to report abstract distances to userspace.
> 

Ok I will send a v3 with abstract_distance dropped.

-aneesh

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