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client-ip=165.204.84.17; helo=satlexmb08.amd.com; pr=C Received: from satlexmb08.amd.com (165.204.84.17) by BL02EPF00021F68.mail.protection.outlook.com (10.167.249.4) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.181.6 via Frontend Transport; Fri, 3 Jul 2026 03:31:47 +0000 Received: from satlexmb10.amd.com (10.181.42.219) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.41; Thu, 2 Jul 2026 22:31:46 -0500 Received: from satlexmb07.amd.com (10.181.42.216) by satlexmb10.amd.com (10.181.42.219) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.41; Thu, 2 Jul 2026 22:31:46 -0500 Received: from [10.85.47.78] (10.180.168.240) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server id 15.2.2562.41 via Frontend Transport; Thu, 2 Jul 2026 22:31:43 -0500 Message-ID: <30fcc836-d013-42ec-a683-86994d307e4c@amd.com> Date: Fri, 3 Jul 2026 09:01:37 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/2] sched: Remove sched_class::balance() To: Aaron Lu , Peter Zijlstra CC: , , , , , , , , , , , References: <20260624121327.190063948@infradead.org> <20260702114919.GA186418@bytedance.com> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <20260702114919.GA186418@bytedance.com> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: R/O+udqfQMiOwE+lgDjcAIcXVDbqKk4zFiG3F6tlvpxll5GddJhK4sl/BNvfCbeNVaSTbzUxQPAmidBbWbjmV/MOaxjr/+EGcuxGcMEXmr0QMHzUxBDuNMutF+bbzREG59TfhzqVPGvj0lebw5b8CX01ptvCki/qWa1VGajAeul/oqVFT2xdCfCdaSz32NZAuzwgg5KjAgo9SB6sw85J/n3EgyI7hp1r1jdCktVM77nhN99YRr5NQ5AZl+qEW66nDJwDO/4xHzhIkBkwpOD9yA1YZWZbHdoL/yx1t86X6u8AaWh2eowfgkCfmJT+2i5+HmGWD41hDlWtsIsQ+nAn+ATX7OAFcWfwYxTyyfvPepwTi+J3p+ce3k2ExgXJL4CxbfpEG3WpKkwEaD3kQmXisCockDad3VZ/A72m1sArZkJ6ps8IQkhW1ucaRQkZHZpn X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 03 Jul 2026 03:31:47.1377 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 3f50defe-22d0-44a7-d0bb-08ded8b39ca3 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb08.amd.com] X-MS-Exchange-CrossTenant-AuthSource: BL02EPF00021F68.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: LV3PR12MB9402 Hi Aaron, On 7/2/2026 5:19 PM, Aaron Lu wrote: > Assume cpuX and cpuY are siblings, it appears the following happened: > > cpuX cpuY > > pick_next_task() > goto restart_multi > > rqX->core_pick = pick_task(rqX) > > pick_task(rqY) > pick_task_fair(rqY) > sched_balance_newidle(rqY) > raw_spin_rq_unlock(rqY) // drops core lock > > pick_next_task() > goto restart_multi > rqY->core_pick = pick_task(rqY) > rqX->core_pick = pick_task(rqX) > > if (rqX->curr == rqX->core_pick) > rqX->core_pick = NULL > > UNLOCK rq_lockp(rqY) > > raw_spin_rq_lock(rqY) > > rqY->core_pick = pick_task(rqY) > > p = rqX->core_pick // NULL > cookie_equals(p, cookie) // NULL deref I also feel doing a balance before the core_cookie is finalized can move tasks wrongly to a particular core, only to make them wait later because the pick converted on a different core cookie. Stealing via balance callback still seems like a good option. Then there is the whole issue of core_pick_seq possibly being updated by another CPU on the core by the time rq_lock is dropped and grabbed again. Afaict, anything that drops the core-wide lock should just do a RETRY_TASK if a new task arrives so everything is re-done withing a single core-wide lock critical-section. Up until this cleanup, only fair and ext used the RETRY_TASK mechanism and fair bypassed the newidle with core-sched enabled so it never used the RETRY_TASK mechanism with core-sched so I'm still skeptical on balance for core-sched being done as part of pick. Maybe Peter will have a way to sort it out after he is back from vacation if he hasn't got it all figured out already ;-) -- Thanks and Regards, Prateek