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From: "Gladyshev Ilya" Message-ID: TLS-Required: No Subject: [PATCH v5 0/2] mm: improve folio refcount scalability To: "Gladyshev Ilya" Cc: "Linus Torvalds" , "Andrew Morton" , ivgorbunov@me.com, Liam.Howlett@oracle.com, apopple@nvidia.com, artem.kuzin@huawei.com, baolin.wang@linux.alibaba.com, foxido@foxido.dev, harry.yoo@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, muchun.song@linux.dev, rppt@kernel.org, surenb@google.com, vbabka@suse.cz, yuzhao@google.com, ziy@nvidia.com, pfalcato@suse.de, kirill@shutemov.name X-Migadu-Flow: FLOW_OUT This is v5 of the series, with no changes to the core idea: - Fix missing BUG_ON checks on the sub_and_check APIs - Replace BUG_ON with WARN_ON_ONCE - Fix virtio-mem incorrect page unfreezing (inc -> init) (Note: This would be caught by the WARN_ON checks during testing.) - Do not loop for an additional cycle during CAS reset Original cover letter posted below: Intro =3D=3D=3D=3D=3D This patch optimizes small file read performance and overall folio refcou= nt scalability by refactoring page_ref_add_unless [core of folio_try_get]. This is alternative approach to previous attempts to fix small read performance by avoiding refcount bumps [1][2]. Overview =3D=3D=3D=3D=3D=3D=3D=3D Current refcount implementation is using zero counter as locked (dead/fro= zen) state, which required CAS loop for increments to avoid temporary unlocks = in try_get functions. These CAS loops became a serialization point for other= wise scalable and fast read side. Proposed implementation separates "locked" logic from the counting, allow= ing the use of optimistic fetch_add() instead of CAS. For more details, pleas= e refer to the commit message of the patch itself. Proposed logic maintains the same public API as before, including all exi= sting memory barrier guarantees. Performance =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D Performance was measured using a simple custom benchmark based on will-it-scale[3]. This benchmark spawns N pinned threads/processes that execute the following loop: `` char buf[] fd =3D open(/* same file in tmpfs */); while (true) { pread(fd, buf, /* read size =3D */ 64, /* offset =3D */0) } `` While this is a synthetic load, it does highlight existing issue and doesn't differ a lot from benchmarking in [2] patch. This benchmark measures operations per second in the inner loop and the results across all workers. Performance was tested on top of v6.15 kernel on two platforms. Since threads and processes showed similar performance = on both systems, only the thread results are provided below. The performance improvement scales linearly between the CPU counts shown. Platform 1: 2 x E5-2690 v3, 12C/12T each [disabled SMT] #threads | vanilla | patched | boost (%) 1 | 1343381 | 1344401 | +0.1 2 | 2186160 | 2455837 | +12.3 5 | 5277092 | 6108030 | +15.7 10 | 5858123 | 7506328 | +28.1 12 | 6484445 | 8137706 | +25.5 /* Cross socket NUMA */ 14 | 3145860 | 4247391 | +35.0 16 | 2350840 | 4262707 | +81.3 18 | 2378825 | 4121415 | +73.2 20 | 2438475 | 4683548 | +92.1 24 | 2325998 | 4529737 | +94.7 Platform 2: 2 x AMD EPYC 9654, 96C/192T each [enabled SMT] #threads | vanilla | patched | boost (%) 1 | 1077276 | 1081653 | +0.4 5 | 4286838 | 4682513 | +9.2 10 | 1698095 | 1902753 | +12.1 20 | 1662266 | 1921603 | +15.6 49 | 1486745 | 1828926 | +23.0 97 | 1617365 | 2052635 | +26.9 /* Cross socket NUMA */ 105 | 1368319 | 1798862 | +31.5 136 | 1008071 | 1393055 | +38.2 168 | 879332 | 1245210 | +41.6 /* SMT */ 193 | 905432 | 1294833 | +43.0 289 | 851988 | 1313110 | +54.1 353 | 771288 | 1347165 | +74.7 [0]: https://lore.kernel.org/lkml/cover.1776350895.git.gorbunov.ivan@h-pa= rtners.com/ [1]: https://lore.kernel.org/linux-mm/CAHk-=3Dwj00-nGmXEkxY=3D-=3DZ_qP6ki= GUziSFvxHJ9N-cLWry5zpA@mail.gmail.com/ [2]: https://lore.kernel.org/linux-mm/20251017141536.577466-1-kirill@shut= emov.name/ [3]: https://github.com/antonblanchard/will-it-scale --- Link to v4: https://lore.kernel.org/linux-mm/df26082871b4c65b2bd38d409026= 237c08572836@linux.dev/ Gladyshev Ilya (1): mm: implement page refcount locking via dedicated bit Gorbunov Ivan (1): mm: drop page refcount zero state semantics drivers/pci/p2pdma.c | 4 +- drivers/virtio/virtio_mem.c | 2 +- include/linux/mm.h | 2 +- include/linux/page-flags.h | 13 ++++++ include/linux/page_ref.h | 68 +++++++++++++++++++++++++----- kernel/liveupdate/kexec_handover.c | 6 +-- lib/test_hmm.c | 4 +- mm/hugetlb.c | 2 +- mm/internal.h | 2 +- mm/memremap.c | 4 +- mm/mm_init.c | 6 +-- mm/page_alloc.c | 4 +- 12 files changed, 88 insertions(+), 29 deletions(-) base-commit: 51cb1aa1250c36269474b8b6ca6b6319e170f5a5 --=20 2.54.0