Re: [LSF/MM/BPF TOPIC] Overhauling hot page detection and promotion based on PTE A bit scanning

[Gregory Price <gourry@gourry.net>]

On Fri, Feb 07, 2025 at 04:10:47PM +0800, Huang, Ying wrote:
> SeongJae Park <sj@kernel.org> writes:
> >
> > I agree the fairness is a thing that we need to aware of.  But IMHO, it is
> > something that the async approach can further be advanced for, not a strict
> > blocker for now.
> 
> Personally, I have no objection to async operations in general.
> However, we may need to find some way to control these async operations
> instead of adding more and more background kthreads blindly.  How to
> charge and constrain the resources used by these async operations is
> important too.  For example, some users may want to bind some async
> operations on some CPUs.
> 
> IMHO, we should think about the requirements and possible solutions
> instead of ignoring the issues.
>

It also concerns me that most every proposal on async promotion ignores
the promotion-node selection problem as if it's a secondary issue.

Async systems fundamentally lack accessor-locality information unless it
is recorded - and recording this information is expensive and/or
heuristically imprecise for memory shared across tasks (two threads in
the same process schedule across sockets).

If we can't agree on a solution to this problem, it undercuts many of
these RFCs which often simply hard-code the target node to "0" because
it's too hard or too expensive to consider the multi-socket scenario.

~Gregory