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

["Huang, Ying" <ying.huang@linux.alibaba.com>]

Ritesh Harjani (IBM) <ritesh.list@gmail.com> writes:

> "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.
>
>> If so, making something conceptually better isn't enough for that.
>>
>
> I think Donet's approach was more towards fixing the problem, then
> making it conceptually better.

To fix a theoretical problem instead of a practical problem?

> So, as of now most of us may not see this
> as a problem, since not many systems have different memory tiers
> attached. But with more widespread CXL adoption and more memory tiers in
> the system, we might require more finer control over auto-numa based
> page migration.

By design, normal NUMA balancing (not memory tiering) should migrate
pages between tiers too.  Because it migrates pages to the node near a
CPU regardless of the memory tiers to optimize NUMA locality.

> But hey, I just wanted to voice out my opinion here. If we think
> changing user visible behavior is going to break existing applications
> and we don't want that - then in that case the reasoning sounds ok to
> me.

---
Best Regards,
Huang, Ying