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

[Shakeel Butt <shakeel.butt@linux.dev>]

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>

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?