From nobody Thu Oct 9 07:17:34 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 ; Thu, 19 Jun 2025 18:26:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.178 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750357599; cv=none; b=R2XEGUXOC6BlftetzSdJ0CXKEzpDkDKcDxwkIq5dlK1D32Fnmgmy1YnZMGSTxirWHgQSNFmyamjCrNLWn3DeTovBGC80iRaVPUBb32Oy8sQYAc7Rk1fsPjYea6aP42Q5FTXOVBk0SKxL+UtobPcsSedqoo2y8CvvHgaYaYdpfiQ= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750357599; c=relaxed/simple; bh=0z0z7BEAOwGXGFZrG5T/T3uQfwCpzUKGWzDdv99j4io=; h=From:To:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=a0Ah7FipW5eJMsKweDzdZiWBlkv7MUEVPOV6jZbUhY2zh3JLTYV7VChl2kqOKF3jPaon5qKkOh9lVSH0MIyauOmQPDiRj6Hr587NomH/Be4m7TvFgfzjrLa9G+DAspStBZ5ab+86T69/fW5Iu5n/ulqYy0294qwatYMWYQajZBc= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=cNRmJIOB; arc=none smtp.client-ip=209.85.210.178 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="cNRmJIOB" Received: by mail-pf1-f178.google.com with SMTP id d2e1a72fcca58-7399a2dc13fso1110447b3a.2 for ; Thu, 19 Jun 2025 11:26:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1750357597; x=1750962397; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:to:from:from:to:cc:subject:date:message-id :reply-to; bh=pdmjI2ThvlwaXhc0+WRx0izpT37E64MmtJ5nQi6GWpw=; b=cNRmJIOBx+WOGya2drEE+ifyqTJTDGVH0Lg8E4+bIOt6cCJe2ubsGygfv6WxNoLM0q BpFlpFvZ0mmwgBb5byRYhvKUBGzIFGzKnkMzJeeX+RGOicovDZQzRcpvCTv97rXo/cpF gUjI8w40ZXb+NJvu6z+Tf8g6GdGJl7Zzui7Vo7xsysfLcqGzm78bbPua5g+oZcmPNRVB O5HGYzSg3J15NukQ2sabPSHKy5j+HZO3Bm+td+Fz5TTN9egG/RYHN2VWHZGA/JoFqs+G 4nM7/F8VvVqUNypHlB1Mqxd3fwAIf6HQHnX8inOxvkY/5WkUyBTwlDWiAHA743Wyljhp Kymw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1750357597; x=1750962397; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=pdmjI2ThvlwaXhc0+WRx0izpT37E64MmtJ5nQi6GWpw=; b=KJr+jK9NTpe2pN9x1naJAsVNDIHvAGnyDEc5yL3LP76whajTPHIXFSBMvVPlzLjjRv f0hXVCer7zeJRJZ65UV1Q0tDXTqzRR+TK1MTpk1D9m3MkQ3zJK7HYx8TLZbwZrPsich3 UQt3q/1fys8jL+XOelV5ON4OB1no/Kx8jmtmOi3gqv5XexKMHDXYt4k6bVOXhutSGaJU HwRIVYr6aglqtTOJrELpijJZwf7uYsBGN9A++Pa/mKUpNkmjih1kE3SGmEkAZHDLwdDr Mg3QbfeEVD8Kzm/hA9TbGzdTp8t3uoLdsBM1IZFuW27vs4cTJC0Tvu7oMA9BGhjTVRui LNaw== X-Gm-Message-State: AOJu0YzaiKc05DTqORgdhjg8tnDf5dGXV6O5O3Twney0UrsBtcS93gg/ oED1+XpV22wplMXZnUA7Ai8spZHZzLCeM1ipuWO5a+Mm0rqU7zsKfSCVgrfIKw== X-Gm-Gg: ASbGncttYTyTtuh1HWSaGt28a3WZ5A2qthA9Vm7QRREqDwvV0x1faCiuEzIGhGIecaR dhgCvRSxpTwgrogoOS/c/NYTi8FLtTMFEjyAUOpSDFm7m85SEcqUob9au72tbe954glT7Ak+W65 bhWk9du+2F33QzlhnI0ZpOpuh3zaiics3UiRtHKKsUwMiV3Y5rKHr9NYUgqz8P21dxMcRQqrc/R NuZeCMhhpN71Fac0zR3RPRXqNdAvpNBfpoIctl3zTqBuapUH4xRcd4/LKLYYv9vdGyG/ntgk/v5 vtLkGoxfHcgPs81cfWzaUIm+bdLOB1A8jcxsaKCG4I5+C7uJBYQbFK3yhNOl6g== X-Google-Smtp-Source: AGHT+IF3u9HCei4YN1dYK2/yBG1uGQwggDbweoS5Kq+r1qY02NKQRAP5o0tPrUqTFS6jzyrPUsA40g== X-Received: by 2002:a05:6a00:2e24:b0:748:3385:a4a with SMTP id d2e1a72fcca58-7489d19b134mr30214375b3a.23.1750357597621; 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] --- 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