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Thu, 2 Apr 2026 06:23:12 -0500 From: K Prateek Nayak To: Thomas Gleixner , Ingo Molnar , "Peter Zijlstra" , Sebastian Andrzej Siewior , Borislav Petkov , Dave Hansen , , Catalin Marinas , Will Deacon , Paul Walmsley , Palmer Dabbelt , Albert Ou , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , "Christian Borntraeger" , Arnd Bergmann , "David Laight" , Samuel Holland CC: Darren Hart , Davidlohr Bueso , =?UTF-8?q?Andr=C3=A9=20Almeida?= , , , , , , K Prateek Nayak Subject: [PATCH v3 0/7] futex: Use runtime constants for futex_hash computation Date: Thu, 2 Apr 2026 11:22:43 +0000 Message-ID: <20260402112250.2138-1-kprateek.nayak@amd.com> X-Mailer: git-send-email 2.43.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Content-Type: text/plain X-Originating-IP: [10.180.168.240] X-ClientProxiedBy: satlexmb08.amd.com (10.181.42.217) To satlexmb07.amd.com (10.181.42.216) X-EOPAttributedMessage: 0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: SN1PEPF0002636C:EE_|SA0PR12MB4349:EE_ X-MS-Office365-Filtering-Correlation-Id: e9c55a4a-adc0-4fa6-c940-08de90aa3ed2 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|1800799024|7416014|82310400026|30052699003|36860700016|18002099003|56012099003|921020|13003099007; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: BR+M2tLWmg2u77rwTTSx4mLWmZ0TOW2hIiuYCzNIL6kf+mBrhxulV3evVph+CozCouIonq93PtQ+QyOuaJSimG30znEQ9ScS+TYUdOGYli1vJc76QZJtPWnwN5cYvNsxmzQkFNayi7XvcRJSD6StH77YwHgw8VSKBF10BnB60LhymjXwLIJpmrxiO+ZMVNgmI3qOOLuQV29inm61QFq7bEf/gc3W85gX4IAKCK7vu1B5bOhxKUlzwM9oEMGpJEg8/J++ZoLh4ITZLoJKf7R/p/CL4/xXmi6Og2z94Sl+nQfRKTbyI566RhO3BanojKPyUtv7Y7qR5O3B7awi1DVJITcyx6nttrSHj/E4XVth/ngLxogs1ffo8IMdP0ujcVM49h6jlfsALt2YIESUFtaBg+zUENGx0x5bZq+h8zVKiYU69uRhSqxPGD3ofzByjHmi X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 Apr 2026 11:23:20.5376 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: e9c55a4a-adc0-4fa6-c940-08de90aa3ed2 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SN1PEPF0002636C.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR12MB4349 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260402_042328_994878_5753935D X-CRM114-Status: GOOD ( 12.37 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org tl;dr This series introduces runtime_const_mask_32() and uses runtime constants for __ro_after_init data in futex_hash() hot path. More information can be found on v2 [1]. Comments that have *not* been addressed in this version ======================================================= Samuel had an observation on v2 that __futex_mask is always of the form ((1 << bits) - 1) /* Only lower bits set; bits > 1. */ and ARM64 and RISC-V can use a single ubfx (ARM64), or slli+srli pattern (RISC-V) for the mask operation respectively but this had the main limitation of runtime_const_mask_32() only working with masks of such form and others would fail runtime_const_init() at boot. RISC-V does generated a "addi + slli" pattern with CONFIG_BASE_SMALL=y where the futex_hash_mask can be computed at compile time. The old scheme is retained for now since it is equivalent to the generated asm for !CONFIG_BASE_SMALL and can handle any arbitrary masks allowing for all future use cases. If there is enough interest, please let me know, and I can look into further optimization to runtime_const_mask_32() based on the current use case for __futex_hash. Testing ======= Apart from x86, which was build and boot tested on baremetal, all the other architectures have been build and boot tested with cross-compile + QEMU with some light sanity testing on each. Patches are based on: git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git master at commit 1086b33a3f64 ("Merge branch into tip/master: 'x86/vdso'") (2026-04-02) Everyone has been Cc'd on the cover-letter and the futex bits for the context. Respective arch maintainers, reviewers, and whoever got lucky with get_maintainer.pl have been Cc'd on their respective arch specific changes. Futex maintainers and the lists will be receiving the whole series (sorry in advance!) --- changelog rfc v2..v3: o Collected Ack from Heiko for s390 bits after folding in their suggested changes (Thanks a ton!) o Reordered Patch 2 and Patch 3 to allow for runtime_const_init() at late_initcall() first before introducing runtime_const_mask_32() on ARM64. (David) o Moved the "&" operation outside the inline asm block on ARM64 and RISC-V which allows the compiler to optimize it further if possible. (David) o Dropped the RFC tag. v2: https://lore.kernel.org/lkml/20260316052401.18910-1-kprateek.nayak@amd.com/ [1] changelog rfc v1..rfc v2: o Use runtime constants to avoid the dereference overheads for dynamically allocated futex_queues. o arch/ side plumbings for runtime_const_mask_32() v1: https://lore.kernel.org/all/20260128101358.20954-1-kprateek.nayak@amd.com/ --- K Prateek Nayak (4): arm64/runtime-const: Use aarch64_insn_patch_text_nosync() for patching arm64/runtime-const: Introduce runtime_const_mask_32() riscv/runtime-const: Introduce runtime_const_mask_32() s390/runtime-const: Introduce runtime_const_mask_32() Peter Zijlstra (3): x86/runtime-const: Introduce runtime_const_mask_32() asm-generic/runtime-const: Add dummy runtime_const_mask_32() futex: Use runtime constants for __futex_hash() hot path arch/arm64/include/asm/runtime-const.h | 32 ++++++++++++++------ arch/riscv/include/asm/runtime-const.h | 22 ++++++++++++++ arch/s390/include/asm/runtime-const.h | 22 +++++++++++++- arch/x86/include/asm/runtime-const.h | 14 +++++++++ include/asm-generic/runtime-const.h | 1 + include/asm-generic/vmlinux.lds.h | 5 ++- kernel/futex/core.c | 42 ++++++++++++++------------ 7 files changed, 107 insertions(+), 31 deletions(-) base-commit: 1086b33a3f644c3bc37abefd699defc45accced1 -- 2.34.1