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

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Message-ID: <d91beb53-e940-e02a-f9ca-3326bf914da7@linux.ibm.com>
Date:   Fri, 2 Sep 2022 10:53:40 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Wei Xu <weixugc@...gle.com>, "Huang, Ying" <ying.huang@...el.com>
Cc:     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

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

>>>>>
>>>>> 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.

 
-aneesh