Re: [PATCH] mm/numa_balancing: allow migrate on protnone reference with MPOL_WEIGHTED_INTERLEAVE policy

[David Hildenbrand (Arm)]

On 9/30/26 09:26, Li Zhe wrote:
> MPOL_WEIGHTED_INTERLEAVE is useful on tiered-memory systems because it
> can seed a workload's new allocations across fast memory and slower
> capacity memory according to a configured ratio.
> 
> That initial placement is useful for workload managers and orchestration
> systems.  They can take the amount of fast memory and slower capacity
> memory on a machine into account before starting a workload, and choose a
> weighted policy that seeds the workload across the tiers at allocation
> time.  This avoids starting from an all-fast or all-slow placement and
> then relying on promotion or demotion to reshape a large working set.
> 
> After those pages have been placed, however, the policy cannot currently
> opt in to migrate-on-fault placement.  set_mempolicy() and mbind()
> reject MPOL_WEIGHTED_INTERLEAVE when MPOL_F_NUMA_BALANCING is specified,
> so memory tiering cannot promote hot pages that were initially placed on
> the slower nodes by the weighted policy.
> 
> Initial placement is only a starting point.  Pages initially allocated on
> fast memory are not necessarily the long-term hot pages, and pages
> initially allocated on slower memory may become hot as the workload's hot
> set changes.  The policy therefore needs to be able to combine weighted
> initial placement with memory tiering's NUMA fault based hot-page
> promotion.

Ok, so memory allocation will respect the weights but balancing will ignore
them? That really sounds rather odd to me.

And I assume that was the reason why we might have disallowed the combination:
it turns a weighted mechanism into an unweighted mechanism.

So are we really sure these semantics that you would essentially set in stone
here are the semantics we want? (ignoring weights)

-- 
Cheers,

David