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Received-SPF: Pass (protection.outlook.com: domain of amd.com designates 165.204.84.17 as permitted sender) receiver=protection.outlook.com; client-ip=165.204.84.17; helo=satlexmb08.amd.com; pr=C Received: from satlexmb08.amd.com (165.204.84.17) by SJ5PEPF000001C9.mail.protection.outlook.com (10.167.242.37) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.569.4 via Frontend Transport; Tue, 27 Oct 2026 08:03:02 +0000 Received: from satlexmb07.amd.com (10.181.42.216) 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 15.2.2562.53; Tue, 27 Oct 2026 03:03:01 -0500 Received: from [10.136.43.232] (10.180.168.240) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server id 15.2.2562.53 via Frontend Transport; Tue, 27 Oct 2026 03:02:55 -0500 Message-ID: <006b2a7f-ae4f-4622-87ed-de76d92847b6@amd.com> Date: Tue, 27 Oct 2026 13:32:54 +0530 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/6] sched/numa: Notify the scheduling class of preferred node changes To: Andrea Righi CC: Tejun Heo , David Vernet , Changwoo Min , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Vladimir Vdovin , Gregory Price , David Dai , Emil Tsalapatis , Christian Loehle , Balbir Singh , Lee Trager , , References: <20261027041536.3317971-1-arighi@nvidia.com> <20261027041536.3317971-4-arighi@nvidia.com> <26d4c1d1-19dd-415e-beb4-05b7f34fecb3@amd.com> Content-Language: en-US From: K Prateek Nayak In-Reply-To: Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: V0NFAtfjplA6t3ytGjVyBT8ovuIhZdV3DmcAwyRSvW7+eMeHHM0PaqbePoVS/DJzlDXxxsMZjAcfD3et5TnId4ZZ/QKsK1x4pc/RKlydt/nn17vgt8lnASHbr6EmcWKgh9qfzW+MtHbrPaIlrnt+ABwTgZIcDNgdoTRL+H18d6fAWTt3nC/3DF7e0cOzQdl9GSBTcxXUW+r+J5MMMOgl7WG6qnEEF121Erh1/JlNHKlyIeqmI4SbpSEUoLg1BMzieU1DBwGB35h8tRzHoE45Z9ckJkAy9hhQMGtponNX2RLyqPnLRWgdPIXrIeT3ZUIwq3svk868ExMWLzgNFCdHCMopRe93Ws7SDbvzjt/RmPBy+/mzPAlVa1cuerlYu8S2M5OgnuNuv2n6buB0z4c1W0iZmQdvOHZASZxXm8LdHfXueFvw7fXCdUYJTDiTD0ia X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 27 Oct 2026 08:03:02.0836 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 6bf9bae5-2050-42b5-a574-08df3400b935 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: SJ5PEPF000001C9.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: IA0PR12MB8423 Hello Andrea, >> On 10/27/2026 9:43 AM, Andrea Righi wrote: >>> --- a/kernel/sched/core.c >>> +++ b/kernel/sched/core.c >>> @@ -8331,8 +8331,13 @@ int migrate_task_to(struct task_struct *p, int target_cpu) >>> void sched_setnuma(struct task_struct *p, int nid) >>> { >>> guard(task_rq_lock)(p); >>> - scoped_guard (sched_change, p, DEQUEUE_SAVE) >>> + scoped_guard (sched_change, p, DEQUEUE_SAVE) { >>> + bool changed = p->numa_preferred_nid != nid; >> >> Don't the callers of sched_setnuma() already check for this? >> >> Only one that doesn't is task_numa_migrate() but >> numa_migrate_preferred() only calls it when >> "task_node(p) == p->numa_preferred_nid" is false and "!sd "case resets >> "p->numa_preferred_nid" to "task_node(p)" when the task cannot be >> migrated. >> >> Afaict, "changed" is always true here. Is there a scenario that I might >> be missing? > > With the current callers it's always true for the tasks that can reach > the callback. > > added the check just because ->numa_node_changed() is a generic > sched_class callback and its contract is that it's invoked only when the > preferred node actually changes. Checking it in sched_setnuma(), under > the rq lock we're already holding, makes sure that the contract is > honored independently of what the callers of sched_setnuma() are doing, > now and in the future, instead of relying on each of them to check it. > > That said, I don't have a strong opinion on that, if we prefer to keep > sched_setnuma() minimal, I can drop the check and document that the > callers must only invoke it on a change. I too don't have a strong opinion but I'm all for smaller diffstat whenever possible ;-) Let us wait on Peter in case he would prefer to have the defensive check. FWIW, I've been running a kernel with a: WARN_ON_ONCE(p->numa_preferred_nid == nid) for a while and I haven't hit any warnings yet from sched_setnuma(). -- Thanks and Regards, Prateek "Program testing can be used to show the presence of bugs, but never to show their absence!" - Edsger W. Dijkstra