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

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Date:   Fri, 02 Sep 2022 13:40:50 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Aneesh Kumar K V <aneesh.kumar@...ux.ibm.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>
Subject: Re: [PATCH v3 updated] mm/demotion: Expose memory tier details via
 sysfs

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.

>>>>>>
>>>>>> A directory hierarchy looks like
>>>>>> :/sys/devices/virtual/memory_tiering$ tree memory_tier4/
>>>>>> memory_tier4/
>>>>>> ├── 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
>>>>>
>>>>> I don't think that it is a good idea to show toptier information in user
>>>>> space interface.  Because it is just a in kernel implementation
>>>>> details.  Now, we only promote pages from !toptier to toptier.  But
>>>>> there may be multiple memory tiers in toptier and !toptier, we may
>>>>> change the implementation in the future.  For example, we may promote
>>>>> pages from DRAM to HBM in the future.
>>>>>
>>>>
>>>>
>>>> In the case you describe above and others, we will always have a list of
>>>> NUMA nodes from which memory promotion is not done.
>>>> /sys/devices/virtual/memory_tiering/toptier_nodes shows that list.
>>>
>>> I don't think we will need that interface if we don't restrict promotion
>>> in the future.  For example, he can just check the memory tier with
>>> smallest number.
>>>
>>> TBH, I don't know why do we need that interface.  What is it for?  We
>>> don't want to expose unnecessary information to restrict our in kernel
>>> implementation in the future.
>>>
>>> So, please remove that interface at least before we discussing it
>>> thoroughly.
>> 
>> I have asked for this interface to allow the userspace to query a list
>> of top-tier nodes as the targets of userspace-driven promotions.  The
>> idea is that demotion can gradually go down tier by tier, but we
>> promote hot pages directly to the top-tier and bypass the immediate
>> tiers.
>> 
>> Certainly, this can be viewed as a policy choice.  Given that now we
>> have a clearly defined memory tier hierarchy in sysfs and the
>> toptier_nodes content can be constructed from this memory tier
>> hierarchy and other information from the node sysfs interfaces, I am
>> fine if we want to remove toptier_nodes and keep the current memory
>> tier sysfs interfaces to the minimal.
>>
>
>
> Ok I can do a v4 with toptier_nodes dropped.

Thanks!

Best Regards,
Huang, Ying