linux-kernel - Re: [PATCH 40/41] mm: separate vma->lock from vm_area

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Message-ID: <CAG48ez1=-tY8nrsX+T=AJBmW05E8sAvZNj80Wev9sGwR9NZysQ@mail.gmail.com>
Date:   Tue, 17 Jan 2023 19:33:59 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, michel@...pinasse.org,
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Subject: Re: [PATCH 40/41] mm: separate vma->lock from vm_area_struct

On Mon, Jan 9, 2023 at 9:55 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
> 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)?