linux-kernel - Re: [RFC PATCH v2 0/2] mm/damon/paddr: Allow interleaving in migrate

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Message-ID: <aFmq5Y1zKov9__zh@gourry-fedora-PF4VCD3F>
Date: Mon, 23 Jun 2025 14:28:37 -0500
From: Gregory Price <gourry@...rry.net>
To: SeongJae Park <sj@...nel.org>
Cc: Bijan Tabatabai <bijan311@...il.com>, damon@...ts.linux.dev,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com,
	matthew.brost@...el.com, joshua.hahnjy@...il.com, rakie.kim@...com,
	byungchul@...com, ying.huang@...ux.alibaba.com, apopple@...dia.com,
	bijantabatab@...ron.com, venkataravis@...ron.com,
	emirakhur@...ron.com, ajayjoshi@...ron.com, vtavarespetr@...ron.com
Subject: Re: [RFC PATCH v2 0/2] mm/damon/paddr: Allow interleaving in
 migrate_{hot,cold} actions

On Fri, Jun 20, 2025 at 01:21:55PM -0700, SeongJae Park wrote:
> Hi Bijan,
> 
> On Fri, 20 Jun 2025 13:04:56 -0500 Bijan Tabatabai <bijan311@...il.com> wrote:
> 
> [...]
> > This patch set adds the mechanism for dynamically changing how application
> > data is interleaved across nodes while leaving the policy of what the
> > interleave weights should be to userspace. It does this by modifying the
> > migrate_{hot,cold} DAMOS actions to allow passing in a list of migration
> > targets to their target_nid parameter. When this is done, the
> > migrate_{hot,cold} actions will migrate pages between the specified nodes
> > using the global interleave weights found at
> > /sys/kernel/mm/mempolicy/weighted_interleave/node<N>. This functionality
> > can be used to dynamically adjust how pages are interleaved by changing the
> > global weights. When only a single migration target is passed to
> > target_nid, the migrate_{hot,cold} actions will act the same as before.
> 
> This means users are required to manipulate two interfaces.  DAMON sysfs for
> target nodes, and weighted_interleave sysfs for weights.  I don't think this
> coupling is very ideal.
> 

Just tossing this out there - weighted interleave sysfs entries *should*
be automatic, and the preferred weights shouldn't really ever change
over time.  Even if they did, if it's the result of devices coming and
going - the updates should also be automatic.

So, in practice, a usually probably only has to twiddle DAMON.

I don't have a strong opinion on whether DAMON should leverage the
mempolicy interface, but I think the way it is designed now is
acceptable.

~Gregory