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Tue, 19 Aug 2025 15:32:35 +0000 Message-ID: Date: Tue, 19 Aug 2025 23:32:25 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] sched/fair: Remove sched_idle_cpu() usages in select_task_rq_fair() To: Christian Loehle , Chengming Zhou CC: , , , , , , , , , References: <20250818124702.163271-1-chengming.zhou@linux.dev> <87212818-1f59-45e4-9a51-dca62ddb9633@arm.com> Content-Language: en-US From: "Chen, Yu C" In-Reply-To: <87212818-1f59-45e4-9a51-dca62ddb9633@arm.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SG2P153CA0007.APCP153.PROD.OUTLOOK.COM (2603:1096::17) To DM4PR11MB6020.namprd11.prod.outlook.com (2603:10b6:8:61::19) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DM4PR11MB6020:EE_|DS0PR11MB7802:EE_ X-MS-Office365-Filtering-Correlation-Id: be5b21cf-2c1d-43eb-d35f-08dddf359ee1 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|376014|7416014|1800799024; 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Look a simple example below. >> >> root >> / \ >> kubepods system >> / \ >> burstable besteffort >> (cpu.idle == 1) >> >> When a sched_idle cpu is just running tasks from besteffort group, >> sched_idle_cpu() will return true in this case, but this cpu pick >> is bad for wakee task from system group. Because the system group >> has lower weight than kubepods, work conservation is somewhat >> broken too. >> >> In a nutshell, sched_idle_cpu() should consider the wakee task group's >> relationship with sched_idle tasks running on the cpu. >> >> Obviously, it's hard to do so. This patch chooses the simple approach >> to remove all sched_idle_cpu() considerations in select_task_rq_fair() >> to bring back work conservation in these cases. > > OTOH sched_idle_cpu() CPUs are guaranteed to not be in an idle state and > potentially already have DVFS on some higher level... > Is it because the schedutil governor considers the utilization of SCHED_IDLE, thus causing schedutil to request a higher frequency? The commit 3c29e651e16d ("sched/fair: Fall back to sched-idle CPU if an idle CPU isn't found") mentions that choosing a CPU running a SCHED_IDLE task can avoid waking a CPU from a deep sleep state. If this is the case, can we say that if an administrator sets the cpufreq governor to "performance" and disables deep idle states, an idle CPU would be more preferable than a CPU running a SCHED_IDLE task? On the other hand, if per_cpu(cpufreq_update_util_data, cpu) is NULL and only shallow idle states are enabled in idle_get_state(), should we skip SCHED_IDLE to achieve work conservation? thanks, Chenyu