Re: Roadmap for extending DAMON beyond pte-accessed bit

[SJ Park <sj@kernel.org>]

On Wed, 24 Jun 2026 07:56:14 -0700 SeongJae Park <sj@kernel.org> wrote:

> On Mon, 25 May 2026 15:52:08 -0700 SeongJae Park <sj@kernel.org> wrote:
> 
> > Hello,
> > 
> > 
> > TLDR: Let's extend DAMON for data attributes monitoring, andd then further
> > extend that for multiple access check primitives including page faults, perf
> > memory-access events and optimized AMD IBS-like h/w feature drivers.
> [...] 
> > Roadmap
> > =======
> > 
> > Assuming you agree to use the data attributes monitoring interface, I suggest
> > us to do the work in below roadmap.
> > 
> > Milestone 1: PTE Accessed bit as one of the data attributes
> > -----------------------------------------------------------
> > 
> > I will work on stabilizing and further extending the data attributes interface
> > and internal framework to be able to support PTE Accessed bit.  This may take
> > no small amount of efforts, but hopefully doable by the end of this year.
> 
> Let me share more detailed plan for this milestone.  Nothing is set on the
> stone.  Everything can be changed on the road.
> 
> The overall goal is, making attributes monitoring be able to do everything we
> are doing now with PTE Accessed-bit based monitoring.  It is needed because we
> want to do everythin we can do now with the PMU/page fault-based DAMON.  Also
> it is needed because we will need to deprecate nr_accesses in long term without
> breaking users.  For this, below changes will be made.
> 
> 1. Support moving sum for probe_hits
> 
> nr_accesses is providing a pseudo moving sum for convenient and fast monitoring
> snapshot retrieval.  Data attributes monitoring results (probe_hits) is not.
> To make attributes monitoring be a foundation for the future PMU-like things
> based data access monitoring, we will first make probe_hits be able to provide
> the psuedo moving sum.  Currently two patch series for this purpose are posted
> on the mailing list:
> 
> - nr_accesses_bp infra cleanup/optimization
>   https://lore.kernel.org/20260622142139.30269-1-sj@kernel.org
> - exposing probe_hits pseudo moving sum
>   https://lore.kernel.org/20260621214231.13449-1-sj@kernel.org
> 
> I aim these to be merged into the next merge window (7.3-rc1).
> 
> 2. Support attrs-only monitoring
> 
> As a next step, we will add an toption to stop doing the PTE Accessed bit but
> only attributes monitoring.  It will allow users to set weights for each probe,
> and do the regions split/merge operations based on the weighted average of the
> probe_hits.  In this way, DAMON can run in two modes: access + attrs and
> attrs-only modes.

As of this writing, two patch series [1,2] for the above two changes are merged
into mm-new.  The mm-new having the second change is not yet pushed to the
repo, but definitely queued [3] for.  Hopefully those will be merged into
7.3-rc1.

> 
> 3. Extend attrs for PTE Accessed bit check
> 
> Further, we will extend DAMON attributes monitoring to handle PTE Accessed bit
> check.  We will extend it to let users configure probing preparation
> operations.  The first operation to support will be the PTE Accessed bit
> clearing operation.  Together, we will introduce new DAMON probe filter that
> checks the PTE Accessed bit.  Using those, users will be able to do the
> PTE Accessed bit based access monitoring in attrs-only monitoring mode.

I have a hacking level prototype for this in damon/next tree.  I will take more
time for stabilizing it.  I'm aiming to make this be merged into 7.4-rc1.

> 
> 4. Extend attrs for DAMOS
> 
> Finally, we will extend DAMOS to be able to run with probe_hits.  That is, it
> will be extended to let users configure the DAMOS target access pattern and do
> its quota-prioritization based on the probe_hits.
> 
> After the four major changes are made, the attributes only mode DAMON can do
> everything current DAAMON is doing, without nr_accesses.  We will then move on
> to the milestone 2 of this project, and start a long term nr_accesses
> deprecation.
> 
> I hope all changes to be landed by 7.5-rc1.

So I believe we are still on the track :)

[1] moving sum: https://lore.kernel.org/20260703170605.94472-1-sj@kernel.org
[2] attrs-only: https://lore.kerrnel.org/20260710134651.18084-1-sj@kernel.org
[3] attrs-only enqueue notice:
    https://lore.kernel.org/20260710202349.2F2DA1F000E9@smtp.kernel.org/


Thanks,
SJ