linux-kernel - Re: [PATCH 2/2 v6] mm/mempolicy: Don't create weight sysfs for memoryless nodes

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Message-ID: <95541985-8d40-4ded-a83e-46203c441640@sk.com>
Date: Tue, 4 Mar 2025 22:03:22 +0900
From: Honggyu Kim <honggyu.kim@...com>
To: Gregory Price <gourry@...rry.net>
Cc: kernel_team@...ynix.com, Joshua Hahn <joshua.hahnjy@...il.com>,
 harry.yoo@...cle.com, ying.huang@...ux.alibaba.com,
 gregkh@...uxfoundation.org, rakie.kim@...com, akpm@...ux-foundation.org,
 rafael@...nel.org, lenb@...nel.org, dan.j.williams@...el.com,
 Jonathan.Cameron@...wei.com, dave.jiang@...el.com, horen.chuang@...ux.dev,
 hannes@...xchg.org, linux-kernel@...r.kernel.org,
 linux-acpi@...r.kernel.org, linux-mm@...ck.org, kernel-team@...a.com,
 yunjeong.mun@...com
Subject: Re: [PATCH 2/2 v6] mm/mempolicy: Don't create weight sysfs for
 memoryless nodes

Hi Gregory,

On 3/4/2025 1:19 AM, Gregory Price wrote:
> On Thu, Feb 27, 2025 at 11:32:26AM +0900, Honggyu Kim wrote:
>>
>> But using N_MEMORY doesn't fix this problem and it hides the entire CXL
>> memory nodes in our system because the CXL memory isn't detected at this
>> point of creating node*.  Maybe there is some difference when multiple
>> CXL memory is detected as a single node.
>>
> 
> Hm, well, the node is "created" during early boot when ACPI tables are
> read and the CFMW are discovered - but they aren't necessarily "online"
> at the time they're created.
> 
> There is no true concept of a "Hotplug NUMA Node" - as the node must be
> created at boot time. (tl;dr: N_POSSIBLE will never change).
> 
> This patch may have been a bit overzealous of us, I forgot to ask
> whether N_MEMORY is set for nodes created but not onlined at boot. So
> this is a good observation.

I didn't want to make more noise but we found many issues again after
getting a new machine and started using it with multiple CXL memory.

> 
> It also doesn't help that this may introduce a subtle race condition.
> 
> If a node exists (N_POSSIBLE) but hasn't been onlined (!N_MEMORY) and
> bandwidth information is reported - then we store the bandwidth info
> but don't include the node in the reduction.  Then if the node comes
> online later, we don't re-trigger reduction.
> 
> Joshua we should just drop this patch for now and work with Honggyu and
> friends separately on this issue.  In the meantime we can stick with
> N_POSSIBLE.
> 
> There are more problems in this space - namely how to handle a system
> whereby 8 CXL nodes are "possible" but the user only configures 2 (as
> described by Hyonggye here).  We will probably need to introduce
> hotplug/node on/offline callbacks to re-configure weights.
> 
> ~Gregory

This work won't take a long time so I think we can submit a patch within 
a few days.

Thanks,
Honggyu