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

[Joshua Hahn]

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? Unfortunately this is also not a great time
to submit new features or behaviors as we are in the middle of a large
merge window, and there are a bunch of fixes that maintainers might be
busy looking into : -(

Regardless, thank you for the fix, Zhe! I don't have a strong
preference on the ordering of this against pghot, but if other folks
are concerned that this could create more churn, I think it would be
reasonable to wait for patches 1 and 3 after we discuss with Bharata.

I hope you have a great day : -)
Joshua