Re: [PATCH 4/6] sched/numa: make free-space tiering promotion bypass configurable

[Gregory Price <gourry@gourry.net>]

On Wed, Oct 14, 2026 at 05:43:45PM +0800, 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 workloads may intentionally
> place memory across CXL-attached memory and DRAM to use aggregate memory
> bandwidth. With the free-space bypass, cold one-shot accesses and bursts
> of hot accesses can refill the fast tier and pull the workload away from
> that distribution whenever DRAM has free space.
> 
> Add kernel.numa_balancing_promote_free_space_bypass. It defaults to 1,
> preserving the existing behavior. When cleared, enough free space in the
> target fast-tier node no longer bypasses the normal hotness and
> rate-limit controls.
>

A global switch for this is a poor solution for this problem.

Also, if the memory was spread out intentionally, it presumably was done
so with a memory policy.  In that case, if the user did not set
MPOL_F_NUMA_BALANCING - that spread is already protected because the
mempolicy overrides the numa balancing fault and no migration occurs.

I don't think this is the right direction.

~Gregory