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([140.113.216.168]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-234396eaf62sm1200745ad.9.2025.05.24.08.55.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 24 May 2025 08:55:40 -0700 (PDT) From: Kuan-Wei Chiu To: paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, akpm@linux-foundation.org Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, jserv@ccns.ncku.edu.tw, Kuan-Wei Chiu , Yu-Chun Lin Subject: [PATCH v2 1/3] lib/math/gcd: Use static key to select implementation at runtime Date: Sat, 24 May 2025 23:55:17 +0800 Message-Id: <20250524155519.1142570-2-visitorckw@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20250524155519.1142570-1-visitorckw@gmail.com> References: <20250524155519.1142570-1-visitorckw@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" On platforms like RISC-V, the compiler may generate hardware FFS instructions even if the underlying CPU does not actually support them. Currently, the GCD implementation is chosen at compile time based on CONFIG_CPU_NO_EFFICIENT_FFS, which can result in suboptimal behavior on such systems. Introduce a static key, efficient_ffs_key, to enable runtime selection between the binary GCD (using ffs) and the odd-even GCD implementation. This allows the kernel to default to the faster binary GCD when FFS is efficient, while retaining the ability to fall back when needed. Co-developed-by: Yu-Chun Lin Signed-off-by: Yu-Chun Lin Signed-off-by: Kuan-Wei Chiu --- lib/math/gcd.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-) diff --git a/lib/math/gcd.c b/lib/math/gcd.c index e3b042214d1b..2898ee0e5add 100644 --- a/lib/math/gcd.c +++ b/lib/math/gcd.c @@ -2,6 +2,7 @@ #include #include #include +#include =20 /* * This implements the binary GCD algorithm. (Often attributed to Stein, @@ -11,16 +12,13 @@ * has decent hardware division. */ =20 +DEFINE_STATIC_KEY_TRUE(efficient_ffs_key); + #if !defined(CONFIG_CPU_NO_EFFICIENT_FFS) =20 /* If __ffs is available, the even/odd algorithm benchmarks slower. */ =20 -/** - * gcd - calculate and return the greatest common divisor of 2 unsigned lo= ngs - * @a: first value - * @b: second value - */ -unsigned long gcd(unsigned long a, unsigned long b) +static unsigned long binary_gcd(unsigned long a, unsigned long b) { unsigned long r =3D a | b; =20 @@ -44,9 +42,15 @@ unsigned long gcd(unsigned long a, unsigned long b) } } =20 -#else +#endif =20 /* If normalization is done by loops, the even/odd algorithm is a win. */ + +/** + * gcd - calculate and return the greatest common divisor of 2 unsigned lo= ngs + * @a: first value + * @b: second value + */ unsigned long gcd(unsigned long a, unsigned long b) { unsigned long r =3D a | b; @@ -54,6 +58,11 @@ unsigned long gcd(unsigned long a, unsigned long b) if (!a || !b) return r; 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([140.113.216.168]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-234396eaf62sm1200745ad.9.2025.05.24.08.55.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 24 May 2025 08:55:43 -0700 (PDT) From: Kuan-Wei Chiu To: paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, akpm@linux-foundation.org Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, jserv@ccns.ncku.edu.tw, Kuan-Wei Chiu , Yu-Chun Lin Subject: [PATCH v2 2/3] riscv: Optimize gcd() code size when CONFIG_RISCV_ISA_ZBB is disabled Date: Sat, 24 May 2025 23:55:18 +0800 Message-Id: <20250524155519.1142570-3-visitorckw@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20250524155519.1142570-1-visitorckw@gmail.com> References: <20250524155519.1142570-1-visitorckw@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The binary GCD implementation depends on efficient ffs(), which on RISC-V requires hardware support for the Zbb extension. When CONFIG_RISCV_ISA_ZBB is not enabled, the kernel will never use binary GCD, as runtime logic will always fall back to the odd-even implementation. 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([140.113.216.168]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-234396eaf62sm1200745ad.9.2025.05.24.08.55.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 24 May 2025 08:55:46 -0700 (PDT) From: Kuan-Wei Chiu To: paul.walmsley@sifive.com, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, akpm@linux-foundation.org Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, jserv@ccns.ncku.edu.tw, Kuan-Wei Chiu , Yu-Chun Lin Subject: [PATCH v2 3/3] riscv: Optimize gcd() performance on RISC-V without Zbb extension Date: Sat, 24 May 2025 23:55:19 +0800 Message-Id: <20250524155519.1142570-4-visitorckw@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20250524155519.1142570-1-visitorckw@gmail.com> References: <20250524155519.1142570-1-visitorckw@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The binary GCD implementation uses FFS (find first set), which benefits from hardware support for the ctz instruction, provided by the Zbb extension on RISC-V. Without Zbb, this results in slower software-emulated behavior. Previously, RISC-V always used the binary GCD, regardless of actual hardware support. This patch improves runtime efficiency by disabling the efficient_ffs_key static branch when Zbb is either not enabled in the kernel (config) or not supported on the executing CPU. This selects the odd-even GCD implementation, which is faster in the absence of efficient FFS. This change ensures the most suitable GCD algorithm is chosen dynamically based on actual hardware capabilities. Co-developed-by: Yu-Chun Lin Signed-off-by: Yu-Chun Lin Signed-off-by: Kuan-Wei Chiu --- arch/riscv/kernel/setup.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c index f7c9a1caa83e..f891eedc3644 100644 --- a/arch/riscv/kernel/setup.c +++ b/arch/riscv/kernel/setup.c @@ -21,6 +21,7 @@ #include #include #include +#include =20 #include #include @@ -51,6 +52,8 @@ atomic_t hart_lottery __section(".sdata") ; unsigned long boot_cpu_hartid; =20 +DECLARE_STATIC_KEY_TRUE(efficient_ffs_key); + /* * Place kernel memory regions on the resource tree so that * kexec-tools can retrieve them from /proc/iomem. While there @@ -361,6 +364,9 @@ void __init setup_arch(char **cmdline_p) =20 riscv_user_isa_enable(); riscv_spinlock_init(); + + if (!IS_ENABLED(CONFIG_RISCV_ISA_ZBB) || !riscv_isa_extension_available(N= ULL, ZBB)) + static_branch_disable(&efficient_ffs_key); } =20 bool arch_cpu_is_hotpluggable(int cpu) --=20 2.34.1