Re: [PATCH v1 1/2] mm/vmscan: balance demotion allocation in alloc_demote_folio()

[Bing Jiao]

On Fri, Jan 09, 2026 at 04:52:28PM -0800, Joshua Hahn wrote:
> On Fri, 9 Jan 2026 23:45:57 +0000 Bing Jiao <bingjiao@xxxxxxxxxx> wrote:
>
> > On Thu, Jan 08, 2026 at 06:14:02PM +0530, Donet Tom wrote:
> > >
> > > On 1/7/26 12:58 PM, Bing Jiao wrote:
> > > > +	/* Randomly select a node from fallback nodes for balanced allocation */
> > > > +	if (allowed_mask) {
> > > > +		mtc->nid = node_random(allowed_mask);
> > >
> > >
> > > This random selection can cause allocations to fall back to distant memory
> > > even when the nearer demotion target has sufficient free memory, correct?
> > > Could this also lead to increased promotion latency?
> >
> > Hi Donet,
> >
> > Thanks for your questions.
> >
> > Yes, the random selection could select a distant node and lead to
> > incresed promotion latency.
> >
> > I just realized that the the fallback allocation should not weighted
> > by a single metric, such as node distance, capacity, free space.
>
> Hello Bing, I hope you are doing well!
>
> Yes -- this is also what I believe, and I think this idea of "how should we
> select demotion / allocation targets" is something that is a difficult problem
> (and one that may not have a single solution that "just works").
>
> It's also a question that I have been thinking about, and what was discussed
> in part at LSFMMBPF last year. At the time, I made some auto-tuning weights [1]
> for weighted interleave based on bandwidth capacity, since the main benefit of
> weighted interleave is to distribute memory accesses across multiple nodes
> to maximize how much bandwidth the system can use at once. A follow-up was to
> think about how these weights could change over time, and what heuristics
> should be used to determine how the weights are selected.
>
> Ultimately, we agreed that the heuristics should probably be delegated to
> userspace, since there are just so many scenarios that could change what
> metrics should take priority. (Jonathan Corbet wrote a great summary of the
> discussion in an LWN article [2])
>
> Coming back to this patchset, I think that all of the ideas above apply
> nicely here as well. What nodes should be selected for demotion and how they
> should be weighted is a difficult question, and one that is probably best
> answered by userspace and what workload they expect to use on their specific
> system.
>
> What I do believe though, is that an unweighted random selection / round-robin
> approach to selecting demotion targets might lead to some unexpected
> performance implications.
>
> > We need a thoroughly study before changing alloc_demote_folio().
>
> So I think this is the way to go : -)
> Although, I'm not actively exploring this at the moment ;)
>
> Please let me know what you think, I hope you have a great day!
> Joshua
>
> [1] https://lore.kernel.org/all/20250109185048.28587-1-joshua.hahnjy@xxxxxxxxx/
> [2] https://lwn.net/Articles/1016842/

Hi Joshua, hope you had a great weekend!

I appreciate you sharing that information. I really enjoyed reading these
articles and discussions.

It makes sense to assume users understand their requirements, but I think
the kernel needs internal heuristics for weight adjustment. Because
users often lack the comprehensive and immediate information necessary
to update their configration in a timely manner, unless the system has
an omniscient administrator who can oversee and (pre)allocate resource
for all tasks running on that system. Therefore, I think it is still
necessary to have kernel on weight adjustment.

I will think more about this and explore it further from userspace,
kernel space, or using a hybrid approach.

Thank you again for the sharing!

Best,
Bing