linux-kernel - Re: [RFC PATCH v2 0/3] mm/damon: Introduce node_target_mem

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Message-ID: <20260130014807.51302-1-sj@kernel.org>
Date: Thu, 29 Jan 2026 17:48:06 -0800
From: SeongJae Park <sj@...nel.org>
To: Ravi Jonnalagadda <ravis.opensrc@...il.com>
Cc: SeongJae Park <sj@...nel.org>,
	damon@...ts.linux.dev,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org,
	akpm@...ux-foundation.org,
	corbet@....net,
	bijan311@...il.com,
	ajayjoshi@...ron.com,
	honggyu.kim@...com,
	yunjeong.mun@...com
Subject: Re: [RFC PATCH v2 0/3] mm/damon: Introduce node_target_mem_bp Quota Goal Metric

On Thu, 29 Jan 2026 13:58:11 -0800 Ravi Jonnalagadda <ravis.opensrc@...il.com> wrote:

> This series introduces a new DAMON quota goal metric, `node_target_mem_bp`,
> designed for controlling memory migration in heterogeneous memory systems
> (e.g., DRAM and CXL memory tiering).
> 
> v1: https://lore.kernel.org/linux-mm/20260123045733.6954-1-ravis.opensrc@gmail.com/T/#u
> 
[...]
> Two-Context Setup for Hot Page Distribution
> ===========================================
> 
> For distributing hot pages between two NUMA nodes (e.g., DRAM node 0 and
> CXL node 1), two DAMON contexts work together:
> 
>     Context 0: monitors node 0, migrate_hot -> node 1
>       goal: node_target_mem_bp, nid=0, target=6000
>       "Migrate hot pages out when node 0 exceeds 60% hot"
> 
>     Context 1: monitors node 1, migrate_hot -> node 0
>       goal: node_target_mem_bp, nid=1, target=4000
>       "Migrate hot pages out when node 1 exceeds 40% hot"
> 
> Each context migrates excess hot pages to the other node. The system
> converges when both nodes reach their target hot memory ratios.

Thank you for adding this example use case!  This is very helpful for
understanding how people can use this feature, and if there is a wrong
assumption.

I think the use case idea is nice and making sense to me.  Nonetheless, I find
a DAMON's devil in the detail.

DAMOS quota autotuning assumes applying the given scheme action more
aggressively (increasing quota) will help increasing the quota goal metric.  In
other words, it believes the aggressiveness (tuned quota size) and the metric
value are proportional.  Hence, for the first context, DAMON will migrate hot
pages of node 0 to node 1, when the hot pages in node 0 is less than 60%, and
start gradually decreasing and eventually stop the migration after hot memory
portion on node 0 reaches and exceeds 60%.  A human readable interpretation of
it would be, "Migrate hot pages out when node 0 not exceeds 60% hot", which
makes no sense for your use case.

To make it work as you described, you may implement another metric representing
the ratio of scheme-uneligible memory on the given node.  Say,
'node_ineligible_mem_bp'?  To borrow your above nice notation, it could be
calculated as below:

    (node_capacity - scheme_eligible_bytes_on_node) / node_capacity

Using this, your above use case could implemented like below:

    Context 0: monitors node 0, migrate_hot -> node 1
      goal: node_ineligible_mem_bp, nid=0, target=4000

    Context 1: monitors node 1, migrate_hot -> node 0
      goal: node_ineligible_mem_bp, nid=1, target=6000

And I'm not very sure if that is really what you want.  For example, if node 0
has 30% hot memory and node 1 has 20% hot memory, no migration will happen.

I think you might want node 0 to have more hot memory, but no more than 60% of
the node.  DAMON-based auto-tuned memory tiering [1], for example, use this
kind of approach.  If that's what you want, you could use node_target_mem_bp
together, like below.

    Context 0: monitors node 0, migrate_hot -> node 1
      goal: node_ineligible_mem_bp, nid=0, target=4000

    Context 1: monitors node 1, migrate_hot -> node 0
      goal: node_target_mem_bp, nid=0, target=6000

I'm not still very confident if I understand what you want, because you
mentioned dynamic weighted interleaving was the major motivation of this
project.  In the case, you might want only hot memory be distributed across
NUMA nodes in a specific ratio.  In the case, you may want the denominator be
"scheme-eligible memory of the system" instead of "node capacity".  To borrow
your notation again,

    scheme_eligible_bytes_on_node / scheme_eligible_bytes_on_system

Let's call this just node_target_mem_bp2.  Then, if you want node 0 and 1 to
have 60% and 40% of hot memory, you could setup DAMOS as below:

    Context 0: monitors node 0, migrate_hot -> node 1
      goal: node_target_mem_bp2, nid=1, target=4000

    Context 1: monitors node 1, migrate_hot -> node 0
      goal: node_target_mem_bp2, nid=0, target=6000

[...]
> Status
> ======
> 
> These patches have been compile-tested but have NOT been tested on actual
> hardware.

It will be very helpful!

> Feedback on the design and approach is appreciated.

So you might need to change the definition and name of the metric, and/or add
new metrics.  But the basic theory of the requirements, the design and the
implementation approach of this patch series looks good to me!

> 
> References
> ==========
> 
> [1] mm/damon/vaddr: Allow interleaving in migrate_{hot,cold} actions
>     https://lore.kernel.org/linux-mm/20250709005952.17776-1-bijan311@gmail.com/

[1] https://github.com/damonitor/damo/blob/next/scripts/mem_tier.sh


Thanks,
SJ

[...]