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

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Message-ID: <CAJuCfpHSZouW44MqdZxpAB_NAdimdU2kcZ2sDo2mfNCf_nmL3w@mail.gmail.com>
Date:   Tue, 17 Jan 2023 11:01:07 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Jann Horn <jannh@...gle.com>
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Subject: Re: [PATCH 40/41] mm: separate vma->lock from vm_area_struct

On Tue, Jan 17, 2023 at 10:34 AM Jann Horn <jannh@...gle.com> wrote:
>
> 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)?

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