Re: [PATCH 0/4] move per-vma lock into vm_area_struct

Suren Baghdasaryan
2024-11-11T21:41:20Z

On Mon, Nov 11, 2024 at 12:55 PM Suren Baghdasaryan <surenb@google.com> 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].
> This patchset moves vm_lock back into vm_area_struct, aligning it at the
> cacheline boundary and changing the cache to be cache-aligned as well.
> 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.
> To minimize memory overhead, vm_lock implementation is changed from
> using rw_semaphore (40 bytes) to an atomic (8 bytes) and several
> vm_area_struct members are moved into the last cacheline, resulting
> in a less fragmented structure:
>
> struct vm_area_struct {
> union {
> struct {
> long unsigned int vm_start; /* 0 8 */
> long unsigned int vm_end; /* 8 8 */
> }; /* 0 16 */
> struct callback_head vm_rcu ; /* 0 16 */
> } __attribute__((__aligned__(8))); /* 0 16 */
> struct mm_struct * vm_mm; /* 16 8 */
> pgprot_t vm_page_prot; /* 24 8 */
> union {
> const vm_flags_t vm_flags; /* 32 8 */
> vm_flags_t __vm_flags; /* 32 8 */
> }; /* 32 8 */
> bool detached; /* 40 1 */
>
> /* XXX 3 bytes hole, try to pack */
>
> unsigned int vm_lock_seq; /* 44 4 */
> struct list_head anon_vma_chain; /* 48 16 */
> /* --- cacheline 1 boundary (64 bytes) --- */
> struct anon_vma * anon_vma; /* 64 8 */
> const struct vm_operations_struct * vm_ops; /* 72 8 */
> long unsigned int vm_pgoff; /* 80 8 */
> struct file * vm_file; /* 88 8 */
> void * vm_private_data; /* 96 8 */
> atomic_long_t swap_readahead_info; /* 104 8 */
> struct mempolicy * vm_policy; /* 112 8 */
>
> /* XXX 8 bytes hole, try to pack */
>
> /* --- cacheline 2 boundary (128 bytes) --- */
> struct vma_lock vm_lock (__aligned__(64)); /* 128 4 */
>
> /* XXX 4 bytes hole, try to pack */
>
> struct {
> struct rb_node rb (__aligned__(8)); /* 136 24 */
> long unsigned int rb_subtree_last; /* 160 8 */
> } __attribute__((__aligned__(8))) shared; /* 136 32 */
> struct vm_userfaultfd_ctx vm_userfaultfd_ctx; /* 168 0 */
>
> /* size: 192, cachelines: 3, members: 17 */
> /* sum members: 153, holes: 3, sum holes: 15 */
> /* padding: 24 */
> /* forced alignments: 3, forced holes: 2, sum forced holes: 12 */
> } __attribute__((__aligned__(64)));
>
> Memory consumption per 1000 VMAs becomes 48 pages, saving 2 pages compared
> to the 50 pages in the baseline:
>
> slabinfo after vm_area_struct changes:
> <name> ... <objsize> <objperslab> <pagesperslab> : ...
> vm_area_struct ... 192 42 2 : ...
>
> Performance measurements using pft test on x86 do not show considerable
> difference, on Pixel 6 running Android it results in 3-5% improvement in
> faults per second.
>
> [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/

And of course I forgot to update Lorenzo's new locking documentation :/
Will add that in the next version.

>
> Suren Baghdasaryan (4):
> mm: introduce vma_start_read_locked{_nested} helpers
> mm: move per-vma lock into vm_area_struct
> mm: replace rw_semaphore with atomic_t in vma_lock
> mm: move lesser used vma_area_struct members into the last cacheline
>
> include/linux/mm.h | 163 +++++++++++++++++++++++++++++++++++---
> include/linux/mm_types.h | 59 +++++++++-----
> include/linux/mmap_lock.h | 3 +
> kernel/fork.c | 50 ++----------
> mm/init-mm.c | 2 +
> mm/userfaultfd.c | 14 ++--
> 6 files changed, 205 insertions(+), 86 deletions(-)
>
>
> base-commit: 931086f2a88086319afb57cd3925607e8cda0a9f
> --
> 2.47.0.277.g8800431eea-goog
>