Re: [PATCH v2 0/4] sched/numa: stop VMA scan filters from gating promotion

[David Hildenbrand (Arm)]

On 9/17/26 08:59, Gregory Price wrote:
> On Wed, Sep 16, 2026 at 10:35:25PM -0700, Andrew Morton wrote:
>> On Thu, 10 Sep 2026 20:18:22 -0400 Gregory Price <gourry@xxxxxxxxxx> wrote:
>>
>>>
>>> Functional observation:
>>>   A 20 GB hash table VMA that previously remained entirely on CXL was
>>>   split evenly between DRAM and CXL after the changes - and tier
>>>   residency tracked hotness.  This was previously affected by the
>>>   stavation issue caused by the "unaccessed VMA" filter.
>>
>> This seems very significant?
>>
>> Why cc:stable and Fixes:?  Is this something which ran at these sorts
>> of speeds before the offending commits?
>>
> 
> It's actually that the changes functionally broke numa balancing to the
> point that it just became completely ineffective for very common use
> cases.
> 
> We only didn't notice because another bug:
> https://lore.kernel.org/all/20260629163337.1264881-1-hannes@xxxxxxxxxxx/
> 
> hid almost all these issues from being apparent on large workloads
> (which was differently horrendous - that bug made ~70-90% of some
>  database workload memory ineligible for tiering)
> 
> Once the shmem fix went in, all the starvation and filtering issues
> become more apparent when the bandwidth numbers stopped making sense
> (see above).
> 
> Basically numa balancing has been broken since 2022/2023, and anyone
> who has done any testing with it since then has been working off of
> bad data (incomplete VMA coverage).

I'll go through the patches from a MM side. I am not a big expert on
kernel/sched/fair.c side of things. Would be good if someone could have a look
on that.

-- 
Cheers,

David