linux-kernel - Re: [RFC PATCH 0/4] mm/damon: Add DAMOS action to interleave data across nodes

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Message-ID: <CAMvvPS7qgos5OFo=W_FftEn=3QqbsxKCR_-3_eiTQeqne_Yj1Q@mail.gmail.com>
Date: Fri, 13 Jun 2025 11:12:56 -0500
From: Bijan Tabatabai <bijan311@...il.com>
To: Rakie Kim <rakie.kim@...com>
Cc: sj@...nel.org, akpm@...ux-foundation.org, corbet@....net, david@...hat.com, 
	ziy@...dia.com, matthew.brost@...el.com, joshua.hahnjy@...il.com, 
	byungchul@...com, gourry@...rry.net, ying.huang@...ux.alibaba.com, 
	apopple@...dia.com, bijantabatab@...ron.com, venkataravis@...ron.com, 
	emirakhur@...ron.com, ajayjoshi@...ron.com, vtavarespetr@...ron.com, 
	linux-mm@...ck.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kernel_team@...ynix.com, damon@...ts.linux.dev
Subject: Re: [RFC PATCH 0/4] mm/damon: Add DAMOS action to interleave data
 across nodes

On Fri, Jun 13, 2025 at 4:55 AM Rakie Kim <rakie.kim@...com> wrote:
>
> On Thu, 12 Jun 2025 13:13:26 -0500 Bijan Tabatabai <bijan311@...il.com> wrote:
> > From: Bijan Tabatabai <bijantabatab@...ron.com>
> >
> > A recent patch set automatically set the interleave weight for each node
> > according to the node's maximum bandwidth [1]. In another thread, the patch
> > set's author, Joshua Hahn, wondered if/how these weights should be changed
> > if the bandwidth utilization of the system changes [2].
> >
> > 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 adding a new
> > DAMOS action: DAMOS_INTERLEAVE. We implement DAMOS_INTERLEAVE with both
> > paddr and vaddr operations sets. Using the paddr version is useful for
> > managing page placement globally. Using the vaddr version limits tracking
> > to one process per kdamond instance, but the va based tracking better
> > captures spacial locality.
>
> Hi Bijan,
>
> Thank you for explaining the motivation and need behind this patch.
> I believe it's important to consider the case where a new memory node
> is added and the interleave weight values are recalculated.
>
> If a new memory node (say, node2) is added, there are two possible
> approaches to consider.
>
> 1. Migrating pages to the newly added node2.
>    In this case, there is a potential issue where pages may be migrated
>    to node2, even though it is not part of the nodemask set by the user.
>
> 2. Ignoring the newly added node2 and continuing to use only the existing
>    nodemask for migrations.
>    However, if the weight values have been updated considering node2
>    performance, avoiding node2 might reduce the effectiveness of using
>    Weighted Interleave.
>
> It would be helpful to consider these two options or explore other
> possible solutions to ensure correctness.
>
> Rakie

Hi Rakie,

Thank you for the reply - this is not a problem I considered, but it
is important.

I think option 2 is the correct choice, and is what is already done by the
policy_nodemask function we use to determine what node to place a page. I
do not think it makes sense to ignore the nodemask when it is explicitly set by
the user.
However, if we decide to change the way we get the interleave weights to be from
a DAMON specific interface, then I think it would make sense to only
migrate to the
newly onlined node if the user sets a weight for that node. This is
because in that
case, the user is explicitly telling DAMON to use that node.

Let me know if you have any other concerns,
Bijan