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Message-ID: <CAJuCfpGwL1rnLRNG=CTfvfi75rsAn-f0mupnZUhm5nDi19Nk-A@mail.gmail.com>
Date: Thu, 9 Jan 2025 07:57:20 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: akpm@...ux-foundation.org, peterz@...radead.org, willy@...radead.org,
liam.howlett@...cle.com, lorenzo.stoakes@...cle.com, mhocko@...e.com,
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mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com,
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lokeshgidra@...gle.com, minchan@...gle.com, jannh@...gle.com,
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linux-doc@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v8 00/16] move per-vma lock into vm_area_struct
On Thu, Jan 9, 2025 at 5:40 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> Btw the subject became rather incomplete given all the series does :)
>
> 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?
That's a good idea. I can amend the cover letter and the description
of patch 11 to include size information.
> 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?
Yes, absolutely. Before this series, vm_area_struct was roughly 168
bytes and vm_lock was 40 bytes. After the changes vm_area_struct
becomes 256 bytes. I was planning to pack the fields as a follow-up
patch similar to an earlier one [1] and bring the size of
vm_area_struct to < 192. I felt this patchset already does many things
and did not include it here but I can add it at the end of this
patchset if you think it's essential.
[1] https://lore.kernel.org/all/20241111205506.3404479-5-surenb@google.com/
>
> > 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@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/20241226170710.1159679-1-surenb@google.com/
> > [5] https://lore.kernel.org/all/20250107030415.721474-1-surenb@google.com/
> > [6] https://lore.kernel.org/all/20241226200335.1250078-1-surenb@google.com/
> >
> > 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(-)
> >
>
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