Re: [PATCH v2] memory tiering: Do not allow promotion if NUMA_BALANCING_MEMORY_TIERING is disabled

[Gregory Price <gourry@gourry.net>]

On Thu, Apr 09, 2026 at 09:12:56AM +0530, Ritesh Harjani wrote:
> "Huang, Ying" <ying.huang@linux.alibaba.com> writes:
> 
> >>>>> Donet Tom <donettom@linux.ibm.com> writes:
> >>
> >>
> >> Thanks for the clarification. I was running some experiments where I
> >> only required migration, not promotion. However, I observed that
> >> promotion was still occurring even when NUMA_BALANCING_MEMORY_TIERING
> >> was disabled, which led me to believe it might be a bug, so I reported
> >> it.
> >>
> >> As I understand it, enabling both NUMA_BALANCING_MEMORY_TIERING and
> >> NUMA_BALANCING_NORMAL results in both promotion and migration. Given
> >> this, do you see any concerns with modifying the behavior of
> >> NUMA_BALANCING_NORMAL?
> >>
> >> With this patch, we would have better control over enabling and
> >> disabling promotion independently. I would appreciate your thoughts on
> >> this.
> >
> > IIUC, we change the existing user visible behavior only with strong
> > enough practical reason.
> 
> So what I understood from this discussion so far is, we don't have any
> mechanism to do auto-numa base page migration between DRAM -to- DRAM w/o
> triggering promotions too from a lower tiers to higher tiers.
> 
> ... This to me sounds more like a broken interface.
> 

It only seems that way because the naming suggests tiering did not exist
prior to _TIERING - instead _NORMAL just operates in a suboptimal manner
when multiple tiers exist.

_NORMAL migrates a page when it detects the node it's on is not the
local node of the task doing the work.

_TIERING takes into account the liveliness of the pages with a
timestamp.

Going to agree with Ying here - this change should be dropped without
additional data.  If you can show _NORMAL would be better off not moving
low-tier pages for at least a handful of common benchmarks, or that
_NORMAL is causes incorrect placement - then I think this change is
warranted.

But as it is, this would just be a behavioral change without supporting
data to justify it.

~Gregory