On Tue, Jan 17, 2023 at 10:34 AM Jann Horn [off-list ref] wrote:

On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan [off-list ref] wrote:

quoted

vma->lock being part of the vm_area_struct causes performance regression
during page faults because during contention its count and owner fields
are constantly updated and having other parts of vm_area_struct used
during page fault handling next to them causes constant cache line
bouncing. Fix that by moving the lock outside of the vm_area_struct.
All attempts to keep vma->lock inside vm_area_struct in a separate
cache line still produce performance regression especially on NUMA
machines. Smallest regression was achieved when lock is placed in the
fourth cache line but that bloats vm_area_struct to 256 bytes.

Just checking: When you tested putting the lock in different cache
lines, did you force the slab allocator to actually store the
vm_area_struct with cacheline alignment (by setting SLAB_HWCACHE_ALIGN
on the slab or with a ____cacheline_aligned_in_smp on the struct
definition)?

Yep, I tried all these combinations and still saw noticeable regression.