Re: [PATCH v6 00/16] move per-vma lock into vm_area_struct

Suren Baghdasaryan
2024-12-16T19:45:15Z

On Mon, Dec 16, 2024 at 11:24 AM 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].
> 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.
> This patchset:
> 1. moves vm_lock back into vm_area_struct, aligning it at the cacheline
> boundary and changing the cache to be cacheline-aligned to minimize
> cacheline sharing;
> 2. changes vm_area_struct initialization to mark new vma as detached until
> it is inserted into vma tree;
> 3. replaces vm_lock and vma->detached flag with a reference counter;
> 4. changes vm_area_struct cache to SLAB_TYPESAFE_BY_RCU to allow for their
> reuse and to minimize call_rcu() calls.
>
> Pagefault microbenchmarks show performance improvement:
> Hmean faults/cpu-1 507926.5547 ( 0.00%) 506519.3692 * -0.28%*
> Hmean faults/cpu-4 479119.7051 ( 0.00%) 481333.6802 * 0.46%*
> Hmean faults/cpu-7 452880.2961 ( 0.00%) 455845.6211 * 0.65%*
> Hmean faults/cpu-12 347639.1021 ( 0.00%) 352004.2254 * 1.26%*
> Hmean faults/cpu-21 200061.2238 ( 0.00%) 229597.0317 * 14.76%*
> Hmean faults/cpu-30 145251.2001 ( 0.00%) 164202.5067 * 13.05%*
> Hmean faults/cpu-48 106848.4434 ( 0.00%) 120641.5504 * 12.91%*
> Hmean faults/cpu-56 92472.3835 ( 0.00%) 103464.7916 * 11.89%*
> Hmean faults/sec-1 507566.1468 ( 0.00%) 506139.0811 * -0.28%*
> Hmean faults/sec-4 1880478.2402 ( 0.00%) 1886795.6329 * 0.34%*
> Hmean faults/sec-7 3106394.3438 ( 0.00%) 3140550.7485 * 1.10%*
> Hmean faults/sec-12 4061358.4795 ( 0.00%) 4112477.0206 * 1.26%*
> Hmean faults/sec-21 3988619.1169 ( 0.00%) 4577747.1436 * 14.77%*
> Hmean faults/sec-30 3909839.5449 ( 0.00%) 4311052.2787 * 10.26%*
> Hmean faults/sec-48 4761108.4691 ( 0.00%) 5283790.5026 * 10.98%*
> Hmean faults/sec-56 4885561.4590 ( 0.00%) 5415839.4045 * 10.85%*
>
> Changes since v5 [4]
> - Added Reviewed-by, per Vlastimil Babka;
> - Added replacement of vm_lock and vma->detached flag with vm_refcnt,
> per Peter Zijlstra and Matthew Wilcox
> - Marked vmas detached during exit_mmap;
> - Ensureed vmas are in detached state before they are freed;
> - Changed SLAB_TYPESAFE_BY_RCU patch to not require ctor, leading to a
> much simpler code;
> - Removed unnecessary patch [5]
> - Updated documentation to reflect changes to vm_lock;
>
> Patchset applies over mm-unstable after reverting v5 of this patchset [4]
> (currently 687e99a5faa5-905ab222508a)

^^^
Please be aware of this if trying to apply to a branch. mm-unstable
contains an older version of this patchset which needs to be reverted
before this one can be applied.

>
> [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/
> [4] https://lore.kernel.org/all/20241206225204.4008261-1-surenb@google.com/
> [5] https://lore.kernel.org/all/20241206225204.4008261-6-surenb@google.com/
>
> Suren Baghdasaryan (16):
> mm: introduce vma_start_read_locked{_nested} helpers
> mm: move per-vma lock into vm_area_struct
> mm: mark vma as detached until it's added into vma tree
> mm/nommu: fix the last places where vma is not locked before being
> attached
> types: move struct rcuwait into types.h
> mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail
> mm: move mmap_init_lock() out of the header file
> mm: uninline the main body of vma_start_write()
> refcount: introduce __refcount_{add|inc}_not_zero_limited
> mm: replace vm_lock and detached flag with a reference count
> mm: enforce vma to be in detached state before freeing
> mm: remove extra vma_numab_state_init() call
> mm: introduce vma_ensure_detached()
> mm: prepare lock_vma_under_rcu() for vma reuse possibility
> mm: make vma cache SLAB_TYPESAFE_BY_RCU
> docs/mm: document latest changes to vm_lock
>
> Documentation/mm/process_addrs.rst | 44 ++++----
> include/linux/mm.h | 162 +++++++++++++++++++++++------
> include/linux/mm_types.h | 37 ++++---
> include/linux/mmap_lock.h | 6 --
> include/linux/rcuwait.h | 13 +--
> include/linux/refcount.h | 20 +++-
> include/linux/slab.h | 6 --
> include/linux/types.h | 12 +++
> kernel/fork.c | 88 ++++------------
> mm/init-mm.c | 1 +
> mm/memory.c | 75 +++++++++++--
> mm/mmap.c | 8 +-
> mm/nommu.c | 2 +
> mm/userfaultfd.c | 31 +++---
> mm/vma.c | 15 ++-
> mm/vma.h | 4 +-
> tools/testing/vma/linux/atomic.h | 5 +
> tools/testing/vma/vma_internal.h | 96 +++++++++--------
> 18 files changed, 378 insertions(+), 247 deletions(-)
>
> --
> 2.47.1.613.gc27f4b7a9f-goog
>