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Notice, find_first_bit() is generally faster than find_nth_bit(), and we employ it when there's a single set bit in the bitmap. See commit 3e061d924fe9c7b4 ("lib/nodemask: optimize node_random for nodemask with single NUMA node"). Signed-off-by: Yury Norov [NVIDIA] Reviewed-by: Thomas Gleixner --- include/linux/find.h | 2 ++ include/linux/nodemask.h | 18 +++--------------- lib/find_bit.c | 24 ++++++++++++++++++++++++ 3 files changed, 29 insertions(+), 15 deletions(-) diff --git a/include/linux/find.h b/include/linux/find.h index 5a2c267ea7f9..98c61838002c 100644 --- a/include/linux/find.h +++ b/include/linux/find.h @@ -44,6 +44,8 @@ unsigned long _find_next_bit_le(const unsigned long *addr= , unsigned long size, unsigned long offset); #endif =20 +unsigned long find_random_bit(const unsigned long *addr, unsigned long siz= e); + #ifndef find_next_bit /** * find_next_bit - find the next set bit in a memory region diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h index 81586d24d248..325e1dd3540b 100644 --- a/include/linux/nodemask.h +++ b/include/linux/nodemask.h @@ -488,21 +488,9 @@ static __always_inline int num_node_state(enum node_st= ates state) static __always_inline int node_random(const nodemask_t *maskp) { #if defined(CONFIG_NUMA) && (MAX_NUMNODES > 1) - int w, bit; - - w =3D nodes_weight(*maskp); - switch (w) { - case 0: - bit =3D NUMA_NO_NODE; - break; - case 1: - bit =3D first_node(*maskp); - break; - default: - bit =3D find_nth_bit(maskp->bits, MAX_NUMNODES, get_random_u32_below(w)); - break; - } - return bit; + int node =3D find_random_bit(maskp->bits, MAX_NUMNODES); + + return node < MAX_NUMNODES ? node : NUMA_NO_NODE; #else return 0; #endif diff --git a/lib/find_bit.c b/lib/find_bit.c index 06b6342aa3ae..d4b5a29e3e72 100644 --- a/lib/find_bit.c +++ b/lib/find_bit.c @@ -18,6 +18,7 @@ #include #include #include +#include =20 /* * Common helper for find_bit() function family @@ -291,3 +292,26 @@ EXPORT_SYMBOL(_find_next_bit_le); #endif =20 #endif /* __BIG_ENDIAN */ + +/** + * find_random_bit - find a set bit at random position + * @addr: The address to base the search on + * @size: The bitmap size in bits + * + * Returns: a position of a random set bit; >=3D @size otherwise + */ +unsigned long find_random_bit(const unsigned long *addr, unsigned long siz= e) +{ + int w =3D bitmap_weight(addr, size); 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Thu, 19 Jun 2025 11:26:35 -0700 (PDT) Received: from localhost ([216.228.127.128]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-3159df846fdsm22075a91.20.2025.06.19.11.26.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Jun 2025 11:26:34 -0700 (PDT) From: Yury Norov To: linux-kernel@vger.kernel.org, "Yury Norov [NVIDIA]" , Rasmus Villemoes , John Stultz , Thomas Gleixner , Stephen Boyd , Andrew Morton Subject: [PATCH v4 2/3] cpumask: introduce cpumask_random() Date: Thu, 19 Jun 2025 14:26:24 -0400 Message-ID: <20250619182626.393541-3-yury.norov@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250619182626.393541-1-yury.norov@gmail.com> References: <20250619182626.393541-1-yury.norov@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" From: "Yury Norov [NVIDIA]" Propagate find_random_bit() to cpumask API. Signed-off-by: Yury Norov [NVIDIA] Reviewed-by: Thomas Gleixner --- include/linux/cpumask.h | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h index 7ae80a7ca81e..39b71b662da3 100644 --- a/include/linux/cpumask.h +++ b/include/linux/cpumask.h @@ -354,6 +354,18 @@ unsigned int cpumask_next_wrap(int n, const struct cpu= mask *src) return find_next_bit_wrap(cpumask_bits(src), small_cpumask_bits, n + 1); } =20 +/** + * cpumask_random - get random cpu in *src. + * @src: cpumask pointer + * + * Return: random set bit, or >=3D nr_cpu_ids if @src is empty. + */ +static __always_inline +unsigned int cpumask_random(const struct cpumask *src) +{ + return find_random_bit(cpumask_bits(src), nr_cpu_ids); +} + /** * for_each_cpu - iterate over every cpu in a mask * @cpu: the (optionally unsigned) integer iterator --=20 2.43.0 From nobody Thu Oct 9 04:52:44 2025 Received: from mail-pf1-f178.google.com (mail-pf1-f178.google.com [209.85.210.178]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6688930E83A for ; 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Thu, 19 Jun 2025 11:26:37 -0700 (PDT) Received: from localhost ([216.228.127.128]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7490a64d289sm344286b3a.132.2025.06.19.11.26.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Jun 2025 11:26:37 -0700 (PDT) From: Yury Norov To: linux-kernel@vger.kernel.org, "Yury Norov [NVIDIA]" , Rasmus Villemoes , John Stultz , Thomas Gleixner , Stephen Boyd , Andrew Morton Subject: [PATCH v4 3/3] clocksource: Improve randomness in clocksource_verify_choose_cpus() Date: Thu, 19 Jun 2025 14:26:25 -0400 Message-ID: <20250619182626.393541-4-yury.norov@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250619182626.393541-1-yury.norov@gmail.com> References: <20250619182626.393541-1-yury.norov@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" From: "Yury Norov [NVIDIA]" The current algorithm of picking a random CPU works OK for dense online cpumask, but if cpumask is non-dense, the distribution of picked CPUs is skewed. For example, on 8-CPU board with CPUs 4-7 offlined, the probability of selecting CPU 0 is 5/8. Accordingly, cpus 1, 2 and 3 are chosen with probability 1/8 each. The proper algorithm should pick each online CPU with probability 1/4. Switch it to cpumask_random(), which has better statistical characteristics. Acked-by: John Stultz Signed-off-by: Yury Norov [NVIDIA] Reviewed-by: Thomas Gleixner --- kernel/time/clocksource.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-) diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 6a8bc7da9062..4b005b2f3ef5 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -342,10 +342,7 @@ static void clocksource_verify_choose_cpus(void) * CPUs that are currently online. */ for (i =3D 1; i < n; i++) { - cpu =3D get_random_u32_below(nr_cpu_ids); - cpu =3D cpumask_next(cpu - 1, cpu_online_mask); - if (cpu >=3D nr_cpu_ids) - cpu =3D cpumask_first(cpu_online_mask); + cpu =3D cpumask_random(cpu_online_mask); if (!WARN_ON_ONCE(cpu >=3D nr_cpu_ids)) cpumask_set_cpu(cpu, &cpus_chosen); } --=20 2.43.0