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

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

"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. 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.


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

Thanks for your feedback!

-ritesh