Re: [PATCH v2 1/3] sched/numa: make free-space tiering promotion bypass configurable

[Li Zhe]

On 10/29/26 12:16 PM, Bharata B Rao wrote:
> On 28-Oct-26 9:42 AM, Li Zhe wrote:
>> Memory tiering normally promotes any recently accessed slow-tier page if
>> the target fast-tier node has enough free space. This preserves the
>> existing policy of filling available fast memory before relying on
>> hotness and rate-limit control.
>>
>> That is not always the desired policy. Some deployments may intentionally
>> keep part of a workload's memory on CXL-attached memory, for example to
>> preserve a desired DRAM/CXL capacity split or bandwidth distribution. With
>> the free-space bypass, short cold accesses and bursts of hot accesses can
>> refill the fast tier and pull the workload away from that distribution
>> whenever DRAM has free space.
> Currently promotion happens on 1st access. Won't it be more natural to tweak
> this frequency of access and also the time between successive accesses to
> prevent promotion due to short cold accesses and bursts of hot accesses
> rather than free space bypass configuration?
I agree that using hotness and rate-limit controls is the more natural
policy here.  That is exactly what this knob is trying to make possible.

The current free-space bypass promotes before reaching those existing
checks.  So when the target fast-tier node has enough free space, short
accesses can still be promoted even if they would not pass the
hotness/rate-limit path.

With this knob disabled, the free-space shortcut is skipped and those
faults fall back to the existing hotness and rate-limit controls instead.
So the patch does not add a new hotness policy; it only makes the
existing one effective even when the fast tier has free space.

Thanks,
Zhe

>
> Regards,
> Bharata.