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Tue, 17 Jun 2025 13:09:06 -0700 (PDT) Received: from localhost ([216.228.127.128]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2365d88be76sm84383085ad.32.2025.06.17.13.09.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Jun 2025 13:09:06 -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 v3 3/3] clocksource: Improve randomness in clocksource_verify_choose_cpus() Date: Tue, 17 Jun 2025 16:08:53 -0400 Message-ID: <20250617200854.60753-4-yury.norov@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20250617200854.60753-1-yury.norov@gmail.com> References: <20250617200854.60753-1-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 selected 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] --- 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