Re: [PATCH v4 5/7] sched/numa: scan PID-inactive VMAs for promotion

[David Hildenbrand (Arm)]

On 9/30/26 13:22, Gregory Price wrote:
> From: "Gregory Price (Meta)" <gourry@xxxxxxxxxx>
> 
> Commit fc137c0ddab2 ("sched/numa: enhance vma scanning logic") skips
> VMAs without recent PID activity. Since only NUMA hint faults record
> that activity, the filter can suppress the fault needed to promote hot
> slow-tier memory.
> 
> Let memory tiering bypass the PID scan filter. In combined mode, use the
> placement decision to restrict top-tier sampling to VMAs that need it,
> while inactive VMAs still receive promotion-only scans.
> 
> Make this decision only when a VMA scan starts. placement_due is
> evaluated for the current task, and a partially scanned VMA is usually
> resumed by a different thread, so re-evaluating it for each chunk would
> demote in-progress placement scans and starve large VMAs of placement
> sampling in multi-threaded processes.
> 
> Track the last completed placement scan separately from scans of any
> kind. Promotion-only scans still update prev_scan_seq, but do not
> advance the placement-starvation horizon.
> 
> On a host with 768 GB of DRAM and 256 GB of CXL memory, one large shmem
> VMA consumed most scanning activity, while 2,537 other VMAs covering
> 84 GB were skipped as inactive. One stand-out result: a hot 20 GB hash
> table ended up trapped entirely on CXL and drove CXL bandwidth
> utilization beyond sustainable levels - resulting in a large regression.
> 
> With this series, the hot hash table ends up split evenly between DRAM
> and CXL, tier residency tracks runtime load, and CXL bandwidth
> utilization drops from 45GB/s (maxed) to 5-10GB/s, while DRAM bandwidth
> utilization increases from ~200GB/s to 250GB/s+, resulting in major
> throughput improvements for the database workload.
> 
> Fixes: fc137c0ddab2 ("sched/numa: enhance vma scanning logic")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: LLM
> Signed-off-by: Gregory Price (Meta) <gourry@xxxxxxxxxx>
> ---

Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David