Re: [PATCH v8 00/16] move per-vma lock into vm_area_struct
From: Suren Baghdasaryan
Date: Thu Jan 09 2025 - 19:17:21 EST
On Thu, Jan 9, 2025 at 7:59 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Thu, Jan 9, 2025 at 5:40 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> >
> > Btw the subject became rather incomplete given all the series does :)
>
> Missed this one. What do you think is worth mentioning here? It's
> true, the patchset does many small things but I wanted to outline the
> main conceptual changes. Please LMK if you think there are more
> changes big enough to be mentioned here.
I just realized that your comment was only about the subject of this
cover letter. Maybe something like this:
per-vma lock and vm_area_struct cache optimizations
Would that be better?
>
> >
> > On 1/9/25 3:30 AM, 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.
> > > 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%*
> >
> > Given how patch 2 discusses memory growth due to moving the lock, should
> > also patch 11 discuss how the replacement with refcount reduces the
> > memory footprint? And/or the cover letter could summarize the impact of
> > the whole series in that aspect? Perhaps the refcount doesn't reduce
> > anything as it's smaller but sits alone in the cacheline? Could it be
> > grouped with some non-hot fields instead as a followup, so could we get
> > to <=192 (non-debug) size without impacting performance?
> >
> > > Changes since v7 [4]:
> > > - Removed additional parameter for vma_iter_store() and introduced
> > > vma_iter_store_attached() instead, per Vlastimil Babka and
> > > Liam R. Howlett
> > > - Fixed coding style nits, per Vlastimil Babka
> > > - Added Reviewed-bys and Acked-bys, per Vlastimil Babka
> > > - Added Reviewed-bys and Acked-bys, per Liam R. Howlett
> > > - Added Acked-by, per Davidlohr Bueso
> > > - Removed unnecessary patch changeing nommu.c
> > > - Folded a fixup patch [5] into the patch it was fixing
> > > - Changed calculation in __refcount_add_not_zero_limited() to avoid
> > > overflow, to change the limit to be inclusive and to use INT_MAX to
> > > indicate no limits, per Vlastimil Babka and Matthew Wilcox
> > > - Folded a fixup patch [6] into the patch it was fixing
> > > - Added vm_refcnt rules summary in the changelog, per Liam R. Howlett
> > > - Changed writers to not increment vm_refcnt and adjusted VMA_REF_LIMIT
> > > to not reserve one count for a writer, per Liam R. Howlett
> > > - Changed vma_refcount_put() to wake up writers only when the last reader
> > > is leaving, per Liam R. Howlett
> > > - Fixed rwsem_acquire_read() parameters when read-locking a vma to match
> > > the way down_read_trylock() does lockdep, per Vlastimil Babka
> > > - Folded vma_lockdep_init() into vma_lock_init() for simplicity
> > > - Brought back vma_copy() to keep vm_refcount at 0 during reuse,
> > > per Vlastimil Babka
> > >
> > > What I did not include in this patchset:
> > > - Liam's suggestion to change dump_vma() output since it's unclear to me
> > > how it should look like. The patch is for debug only and not critical for
> > > the rest of the series, we can change the output later or even drop it if
> > > necessary.
> > >
> > > [1] https://lore.kernel.org/all/20230227173632.3292573-34-surenb@xxxxxxxxxx/
> > > [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/
> > > [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@xxxxxxxxxxxxxx/
> > > [4] https://lore.kernel.org/all/20241226170710.1159679-1-surenb@xxxxxxxxxx/
> > > [5] https://lore.kernel.org/all/20250107030415.721474-1-surenb@xxxxxxxxxx/
> > > [6] https://lore.kernel.org/all/20241226200335.1250078-1-surenb@xxxxxxxxxx/
> > >
> > > Patchset applies over mm-unstable after reverting v7
> > > (current SHA range: 588f0086398e - fb2270654630)
> > >
> > > 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: introduce vma_iter_store_attached() to use with attached vmas
> > > mm: mark vmas detached upon exit
> > > 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/debug: print vm_refcnt state when dumping the vma
> > > mm: remove extra vma_numab_state_init() call
> > > 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 | 152 ++++++++++++++++++++++-------
> > > include/linux/mm_types.h | 36 ++++---
> > > 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 | 128 +++++++++++-------------
> > > mm/debug.c | 12 +++
> > > mm/init-mm.c | 1 +
> > > mm/memory.c | 94 +++++++++++++++---
> > > mm/mmap.c | 3 +-
> > > mm/userfaultfd.c | 32 +++---
> > > mm/vma.c | 23 ++---
> > > mm/vma.h | 15 ++-
> > > tools/testing/vma/linux/atomic.h | 5 +
> > > tools/testing/vma/vma_internal.h | 93 ++++++++----------
> > > 18 files changed, 435 insertions(+), 260 deletions(-)
> > >
> >