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

[Gregory Price <gourry@gourry.net>]

On Wed, Sep 30, 2026 at 10:02:23PM +0800, Li Zhe wrote:
> The use case is closer to the second one, but the main motivation is not
> only startup-time performance.  The more important point is that each
> workload has its own DDR/CXL budget assigned by the workload manager.
> 

mempolicy is the wrong interface to do budget policy, you'd be better
off looking at Joshua's memcg tiered node solutions for that.

> MPOL_WEIGHTED_INTERLEAVE is useful because it lets new allocations
> follow that per-workload budget from the beginning, instead of placing
> everything on one tier first and correcting the placement later. This is
> important for workload orchestration because the workload starts from a
> placement close to its assigned DDR/CXL ratio.
> 

This however is reasonable to me - you'd prefer to spread out the cost
of initial faulting placement explicitly, rather than simply take
fallbacks when the top-tier budget becomes pressured.

i.e. w/o interleave:

[node 0       ]
^^^^^^^^^^^^^^ alloc until full
                vvvvvvvvv fallback
                [node 1          ]

in this scenario you end up with considerable hot memory
on the remote node consolidated in time-space (everything
allocated after node0 becomes full skews heavily toward
node1)

Tiering then likely takes many faults to rebalance after
you've already reached node0 limits.

w/ interleave

[node 0                                      ]
 ^^^vv^^^vv^^^vv^^^vv^^^vv^^^vv....
[node 1                                      ]

In this scenario you do an initial fill distributed by weight
and then let tiering figure it out without consolidating all
of the pressure to the point where node0 has no space left.

That said - this seems like mostly an initial-fill problem, after
you initially fill your memory, a new allocation largely implies
the memory is hot - and you probably prefer that to be local.

So while it's possible to enable this, I'm struggling to understand
the justification.  I think I would like to see numbers that demonstrate
that this is actually beneficial over the life of a workload.

~Gregory