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dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; Received: from LV3PR12MB9356.namprd12.prod.outlook.com (2603:10b6:408:20c::21) by DS0PR12MB8441.namprd12.prod.outlook.com (2603:10b6:8:123::8) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.592.9; Thu, 5 Nov 2026 13:27:18 +0000 Received: from LV3PR12MB9356.namprd12.prod.outlook.com ([fe80::1c36:31b4:c420:6286]) by LV3PR12MB9356.namprd12.prod.outlook.com ([fe80::1c36:31b4:c420:6286%4]) with mapi id 15.21.0592.009; Thu, 5 Nov 2026 13:27:17 +0000 Date: Thu, 5 Nov 2026 08:27:16 -0500 From: Yury Norov To: Fushuai Wang Cc: agordeev@linux.ibm.com, arighi@nvidia.com, borntraeger@linux.ibm.com, bsegall@google.com, corbet@lwn.net, dakr@kernel.org, dietmar.eggemann@arm.com, driver-core@lists.linux.dev, gor@linux.ibm.com, gregkh@linuxfoundation.org, hca@linux.ibm.com, johan@kernel.org, juri.lelli@redhat.com, kprateek.nayak@amd.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, linux@rasmusvillemoes.dk, meted@linux.ibm.com, mgorman@suse.de, mingo@redhat.com, peterz@infradead.org, rafael@kernel.org, rdunlap@infradead.org, rostedt@goodmis.org, skhan@linuxfoundation.org, sshegde@linux.ibm.com, svens@linux.ibm.com, tglx@kernel.org, vincent.guittot@linaro.org, vschneid@redhat.com, yury.norov@gmail.com Subject: Re: [PATCH 3/6] sched: Restore preferred CPUs when hotplug removes the last one Message-ID: References: <20261101192203.6979-4-ynorov@nvidia.com> <20261105090505.93940-1-fushuai.wang@linux.dev> Content-Type: text/plain; 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> > > > +#ifdef CONFIG_PREFERRED_CPU > > + if (cpumask_empty(cpu_preferred_mask)) { > > + pr_notice("sched: last preferred CPU%u offlined; make all active CPUs preferred\n", > > + cpu); > > + reset_preferred_cpumask(); > > Hi, Yury > > If the CPU being offlined is the only preferred CPU, after > reset_preferred_cpumask(), the steal governor may still need > some time to tighten the mask again, and eventually only one > preferred CPU may remain. Would this introduce additional > task migration overhead? This is the safest fallback from the scheduler POV. Current steal governor implementation would require multiple iterations to get back to the old equilibrium. If it will show up as a real problem, we can teach steal governor to converge faster, and/or detect the preferred mask reset. Thanks, Yury