linux-kernel - Re: [PATCH v5 6/9] mm/demotion: Add support for removing node from demotion memory tiers

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Date:   Wed, 8 Jun 2022 13:59:54 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Ying Huang <ying.huang@...el.com>, linux-mm@...ck.org,
        akpm@...ux-foundation.org
Cc:     Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Brice Goglin <brice.goglin@...il.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>,
        Feng Tang <feng.tang@...el.com>,
        Jagdish Gediya <jvgediya@...ux.ibm.com>,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v5 6/9] mm/demotion: Add support for removing node from
 demotion memory tiers

On 6/8/22 1:53 PM, Ying Huang wrote:
> On Wed, 2022-06-08 at 13:50 +0530, Aneesh Kumar K V wrote:
>> On 6/8/22 12:29 PM, Ying Huang wrote:
>>> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
>>>> This patch adds the special string "none" as a supported memtier value
>>>> that we can use to remove a specific node from being using as demotion target.
>>>>
>>>> For ex:
>>>> :/sys/devices/system/node/node1# cat memtier
>>>> 1
>>>> :/sys/devices/system/node/node1# cat ../../memtier/memtier1/nodelist
>>>> 1-3
>>>> :/sys/devices/system/node/node1# echo none > memtier
>>>> :/sys/devices/system/node/node1#
>>>> :/sys/devices/system/node/node1# cat memtier
>>>> :/sys/devices/system/node/node1# cat ../../memtier/memtier1/nodelist
>>>> 2-3
>>>> :/sys/devices/system/node/node1#
>>>
>>> Do you have a practical use case for this?  What kind of memory node
>>> needs to be removed from memory tiers demotion/promotion?
>>>
>>
>> This came up in our internal discussion. It was mentioned that there is
>> a need to skip some slow memory nodes from participating in demotion.
> 
> Again, can you provide a practical use case?  Why we shouldn't demote
> cold pages to these slow memory nodes?  How do we use these slow memory
> node?  These slow memory node is slower than disk?
> 

This was discussed in the context of memory borrowed from remote machine 
(aka OpenCAPI memory). In such case, we would have a memory only NUMA 
node which we want to avoid using for demotion.

-aneesh