Re: [PATCH v4 2/5] mm: move per-vma lock into vm_area_struct

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Nov 20, 2024 at 4:05 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Wed, Nov 20, 2024 at 03:44:29PM -0800, Suren Baghdasaryan wrote:
> > On Wed, Nov 20, 2024 at 3:33 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> > >
> > > On Tue, Nov 19, 2024 at 04:08:23PM -0800, Suren Baghdasaryan wrote:
> > > > Back when per-vma locks were introduces, vm_lock was moved out of
> > > > vm_area_struct in [1] because of the performance regression caused by
> > > > false cacheline sharing. Recent investigation [2] revealed that the
> > > > regressions is limited to a rather old Broadwell microarchitecture and
> > > > even there it can be mitigated by disabling adjacent cacheline
> > > > prefetching, see [3].
> > > > Splitting single logical structure into multiple ones leads to more
> > > > complicated management, extra pointer dereferences and overall less
> > > > maintainable code. When that split-away part is a lock, it complicates
> > > > things even further. With no performance benefits, there are no reasons
> > > > for this split. Merging the vm_lock back into vm_area_struct also allows
> > > > vm_area_struct to use SLAB_TYPESAFE_BY_RCU later in this patchset.
> > > > Move vm_lock back into vm_area_struct, aligning it at the cacheline
> > > > boundary and changing the cache to be cacheline-aligned as well.
> > > > With kernel compiled using defconfig, this causes VMA memory consumption
> > > > to grow from 160 (vm_area_struct) + 40 (vm_lock) bytes to 256 bytes:
> > > >
> > > >     slabinfo before:
> > > >      <name>           ... <objsize> <objperslab> <pagesperslab> : ...
> > > >      vma_lock         ...     40  102    1 : ...
> > > >      vm_area_struct   ...    160   51    2 : ...
> > > >
> > > >     slabinfo after moving vm_lock:
> > > >      <name>           ... <objsize> <objperslab> <pagesperslab> : ...
> > > >      vm_area_struct   ...    256   32    2 : ...
> > > >
> > > > Aggregate VMA memory consumption per 1000 VMAs grows from 50 to 64 pages,
> > > > which is 5.5MB per 100000 VMAs. Note that the size of this structure is
> > > > dependent on the kernel configuration and typically the original size is
> > > > higher than 160 bytes. Therefore these calculations are close to the
> > > > worst case scenario. A more realistic vm_area_struct usage before this
> > > > change is:
> > > >
> > > >      <name>           ... <objsize> <objperslab> <pagesperslab> : ...
> > > >      vma_lock         ...     40  102    1 : ...
> > > >      vm_area_struct   ...    176   46    2 : ...
> > > >
> > > > Aggregate VMA memory consumption per 1000 VMAs grows from 54 to 64 pages,
> > > > which is 3.9MB per 100000 VMAs.
> > > > This memory consumption growth can be addressed later by optimizing the
> > > > vm_lock.
> > > >
> > > > [1] https://lore.kernel.org/all/20230227173632.3292573-34-surenb@google.com/
> > > > [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/
> > > > [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@mail.gmail.com/
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> > >
> > > Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
> >
> > Thanks!
> >
> > >
> > >
> > > One question below.
> > >
> > > > --- a/include/linux/mm_types.h
> > > > +++ b/include/linux/mm_types.h
> > > > @@ -716,8 +716,6 @@ struct vm_area_struct {
> > > >        * slowpath.
> > > >        */
> > > >       unsigned int vm_lock_seq;
> > > > -     /* Unstable RCU readers are allowed to read this. */
> > > > -     struct vma_lock *vm_lock;
> > > >  #endif
> > > >
> > > >       /*
> > > > @@ -770,6 +768,10 @@ struct vm_area_struct {
> > > >       struct vma_numab_state *numab_state;    /* NUMA Balancing state */
> > > >  #endif
> > > >       struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
> > > > +#ifdef CONFIG_PER_VMA_LOCK
> > > > +     /* Unstable RCU readers are allowed to read this. */
> > > > +     struct vma_lock vm_lock ____cacheline_aligned_in_smp;
> > > > +#endif
> > > >  } __randomize_layout;
> > >
> > > Do we just want 'struct vm_area_struct' to be cacheline aligned or do we
> > > want 'struct vma_lock vm_lock' to be on a separate cacheline as well?
> >
> > We want both to minimize cacheline sharing.
> >
>
> For later, you will need to add a pad after vm_lock as well, so any
> future addition will not share the cacheline with vm_lock. I would do
> something like below. This is a nit and can be done later.
>
> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> index 7654c766cbe2..5cc4fff163a0 100644
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -751,10 +751,12 @@ struct vm_area_struct {
>  #endif
>         struct vm_userfaultfd_ctx vm_userfaultfd_ctx;
>  #ifdef CONFIG_PER_VMA_LOCK
> +       CACHELINE_PADDING(__pad1__);
>         /* Unstable RCU readers are allowed to read this. */
> -       struct vma_lock vm_lock ____cacheline_aligned_in_smp;
> +       struct vma_lock vm_lock;
> +       CACHELINE_PADDING(__pad2__);
>  #endif
> -} __randomize_layout;
> +} __randomize_layout ____cacheline_aligned_in_smp;

I thought SLAB_HWCACHE_ALIGN for vm_area_cachep added in this patch
would have the same result, no?

>
>  #ifdef CONFIG_NUMA
>  #define vma_policy(vma) ((vma)->vm_policy)