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

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Message-ID: <Z8XWqQdPC7245FA2@gourry-fedora-PF4VCD3F>
Date: Mon, 3 Mar 2025 11:19:53 -0500
From: Gregory Price <gourry@...rry.net>
To: Honggyu Kim <honggyu.kim@...com>
Cc: Joshua Hahn <joshua.hahnjy@...il.com>, harry.yoo@...cle.com,
	ying.huang@...ux.alibaba.com, kernel_team@...ynix.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

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.

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