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Date:   Fri, 11 Mar 2022 09:39:52 +0100
From:   Oscar Salvador <osalvador@suse.de>
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Abhishek Goel <huntbag@linux.vnet.ibm.com>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm: Only re-generate demotion targets when a numa
 node changes its N_CPU state
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References: <20220310120749.23077-1-osalvador@suse.de>
 <87mthxb514.fsf@yhuang6-desk2.ccr.corp.intel.com>
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On Fri, Mar 11, 2022 at 10:24:07AM +0800, Huang, Ying wrote:
> It may be unnecessary to be fixed in this patch.  But I think we need to
> cleanup the kernel config dependencies of the demotion code at some time.

I am glad you brought this up because it is something I have been
thinking about.
I already added it in my to-do list, but I would do it in a separate
patch if you do not mind.

Now, let us try to untangle this mess:

> 1. Even if !defined(CONFIG_HOTPLUG_CPU) &&
>    !defined(CONFIG_MEMORY_HOTPLUG), we still need to allocate
>    "node_demotion" and call set_migration_target_nodes() during boot time.
> 
> 2. If !defined(CONFIG_MEMORY_HOTPLUG), we don't need
>    migrate_on_reclaim_callback().
> 
> 3. We need defined(CONFIG_NUMA) && defined(CONFIG_MIGRATION) for all
>    these code.

Back in the early versions [1] I asked whether we could have some
scenario where this feature could be used when !CONFIG_MEMORY_HOTPLUG
[2].
The reason I was given is that in order to bind the expose PMEM memory
as RAM (add_memory_driver_managed()), we need MEMORY_HOTPLUG.

Now, as I said back then, I am not sure whether PMEM memory can be
exposed as RAM by other means, but as it was pointed out back then,
it really looks like we, at least, need CONFIG_MEMORY_HOTPLUG.

Ok, so we have our first dependency: CONFIG_MEMORY_HOTPLUG.

Now, about CONFIG_HOTPLUG_CPU, it seems that that is not a strong dependency,
as we do not need cpu-hotplug in order to use the feature.

We definitely need CONFIG_MIGRATION and CONFIG_NUMA though.

So, we have something like:

- Depends:
  * CONFIG_NUMA           (obvius)
  * CONFIG_MIGRATION      (to migrate between nodes)
  * CONFIG_MEMORY_HOTPLUG (to expose PMEM as RAM)

Sounds about right?

[1] https://patchwork.kernel.org/project/linux-mm/patch/20210401183221.977831DE@viggo.jf.intel.com/#24099405
[2] https://patchwork.kernel.org/project/linux-mm/patch/20210401183221.977831DE@viggo.jf.intel.com/#24103467

-- 
Oscar Salvador
SUSE Labs