Re: [PATCH v2 2/3] mm/khugepaged: scan mm's more efficiently under mmap_lock contention

[Baolin Wang]

On 11/6/26 4:57 AM, Lorenzo Stoakes (ARM) wrote:
> khugepaged scans VMAs in collapse_scan_mm_slot() in the same mm until it is
> about to be torn down, prctl(PR_SET_THP_DISABLE) disables THP for it,
> mm->mmap_lock contention occurs, or all its VMAs have been scanned.
>
> When lock contention causes the function to bail out, khugepaged doesn't
> keep track of where it left off, so when the mm is next scanned, it starts
> from address 0 again.
>
> If there's a lot of mmap_lock contention this can significantly slow down
> collapse later in the mm, since the /sys/.../pages_to_scan budget can run
> out before the end of the mm is reached.
>
> The budget is incremented (as tracked in cc->progress) at a rate of 1 page
> per ineligible VMA, but the budget is only checked once eligible VMAs are
> encountered.
>
> Then, it only takes the combined total of ineligible VMAs and eligible but
> non-collapsible pages to exceed the /sys/.../pages_to_scan budget for the
> rest of the mm to be skipped.
>
> For a system with 4 KiB page size and default sysctl settings this only
> requires 4096 ineligible VMAs/uncollapsible pages to be skipped before this
> happens.
>
> Resolve the issue by simply tracking the last examined address in the
> khugepaged_mm_slot object, so when the mm is resumed, the scan starts from
> where it left off.
>
> Additionally, skip to the next candidate if a VMA cannot be collapsed.
>
> This issue was noticed during some work in which an mm was instantiated
> with a large number of small VMAs under significant mmap_lock contention,
> during which collapse was observed to take 10.8s. With this patch applied
> it took 3.6s to collapse the same mm.

Nice results!

> Also, while here, update a couple VM_BUG_ON()'s to VM_WARN_ON_ONCE()'s.
>
> This patch is also a prerequisite for the next patch in the series.
>
> Reviewed-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> Reviewed-by: Lance Yang <lance.yang@xxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> ---

Make sense to me.
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>