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

[IBM]

Gregory Price <gourry@xxxxxxxxxx> writes:

> On Thu, Apr 09, 2026 at 09:12:56AM +0530, Ritesh Harjani wrote:
>> "Huang, Ying" <ying.huang@xxxxxxxxxxxxxxxxx> writes:
>> 
>> >>>>> Donet Tom <donettom@xxxxxxxxxxxxx> 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.
>

Sure Gregory and Ying, got the point.
Thanks for your comments and feedback. Appreciate it!

For now let's live with that, in case if we come across a real world
usecase where we would want, only-migration w/o tiering behavior, then
will share it.

-ritesh