Re: [PATCH v2 2/3] sched/numa: do not feed tiering promotions into task placement stats

[Bharata B Rao]

On 28-Oct-26 8:33 PM, Joshua Hahn wrote:
> On Wed, Oct 28, 2026 at 12:14 AM Li Zhe <lizhe.67@xxxxxxxxxxxxx> wrote:
>>
>> NUMA balancing has separate mode bits for normal task placement and
>> memory tiering. In memory-tiering-only mode, NUMA hinting faults on
>> slow-tier pages are used to detect hot pages and promote them to a top
>> tier. They are not normal task-placement faults.
>>
>> For slow-tier folios, memory tiering reuses the folio last-cpupid field
>> to store access-time metadata. numa_migrate_check() therefore cannot
>> return a real cpupid for those faults. Before this change, it returned the
>> same all-ones last_cpupid value that normal AutoNUMA uses for a first or
>> unknown access.
>>
>> That can make a memory-tiering-only promotion look like an ordinary first
>> private AutoNUMA fault. After a successful promotion, do_numa_page()
>> reports the target top-tier node to task_numa_fault(), so the existing
>> slow-node skip no longer applies. task_numa_fault() then accounts the
>> fault in p->numa_faults and p->numa_faults_locality, and
>> task_numa_placement() can consume that sample to update normal task
>> placement state even though normal NUMA balancing is disabled. In other
>> words, hot-page promotion feedback can leak into the CPU placement
>> heuristic.
> 
> Hi Zhe,
> 
> Thanks for the revision. To me, this looks more like a fix than an
> optimization, since there really is incorrect NUMA balancing behavior
> happening that could be visible in userspace (unintended
> task-placement and related accounting). On the other hand patch 3/3
> doesn't seem like a fix to me despite the name, since this one would
> mitigate the incorrect behavior.
> 
> How about we pull this one out of the series and submit it
> independently with an appropriate Fixes: tag and cc:stable, and
> discuss 1 and 3 later on?

Currently only successful promotions feed stats to scan period update.
This change stops that and nothing updates the scan period for tiering
mode after this change.

Though the direction of the fix is right, I think this can't be an
independent fix until there is some alternative which patch 3 in this
series brings in.

Also it would be good to get a view from Ying Huang as to why only
successful promotions drove the fault stats while unsuccessful promotions
didn't in the tiering mode.

Regards,
Bharata.