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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 SJ1PEPF000023D5.mail.protection.outlook.com (10.167.244.70) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9654.0 via Frontend Transport; Mon, 16 Feb 2026 03:14:45 +0000 Received: from SATLEXMB04.amd.com (10.181.40.145) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.2.2562.17; Sun, 15 Feb 2026 21:14:45 -0600 Received: from satlexmb08.amd.com (10.181.42.217) by SATLEXMB04.amd.com (10.181.40.145) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.39; Sun, 15 Feb 2026 21:14:45 -0600 Received: from [10.136.47.100] (10.180.168.240) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server id 15.2.2562.17 via Frontend Transport; Sun, 15 Feb 2026 21:14:41 -0600 Message-ID: <3eefd122-22e5-4ac1-bcaf-fd7016c77bf2@amd.com> Date: Mon, 16 Feb 2026 08:44:40 +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 0/4] sched: Various reweight_entity() fixes To: Shubhang Kaushik , Peter Zijlstra , CC: , , , , , , , , , , , References: <20260130093439.803225718@infradead.org> <13dff84d-8e40-0248-971f-e731dfb18d70@os.amperecomputing.com> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <13dff84d-8e40-0248-971f-e731dfb18d70@os.amperecomputing.com> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: qyWWPMoxO0AsAoA3JIrnlQGa2EsbN0eIPYY8c1IOFH5NygwWDY7qJ6plPXjAvvDcDQY/H2M1iPKqFNZHypdkP/XuLbcYDaBNsi0rjwoPWKAuDk1Cl+r6b26V9pbacdAamvf97aMD/3nLhKjexkuUP23LqcG0HwgJpjAYl7B0NwASm1UFCqqErOeKnsSkoe0coS5CBVZkPR6HO69Wj+QTCOn6CBPQyqa7cizPgXylXxNcAiU0Dkpn9hPfThdU0okif1nsTcRBQ0ObVCJMKKXkXahaGAFDVtDnzwzyp29YkQzlaMCbfqDb2c34D6m84ptmUdsYPHL6GC5AWFcOPn0/lWjm4MrK0pQgARylJ30vqNGEnQv6geixtc1o7ZlGqcxt7ECxMJAQHqKRkzfuLku7yfFAjHEAWw+cYYFfKtB+KXYWDOOfRsdv6b1B/Z07ZS+L X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 16 Feb 2026 03:14:45.7293 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 9c47a97e-85af-431f-d92c-08de6d098945 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: SJ1PEPF000023D5.namprd21.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR12MB9121 Hello Shubhang, On 2/14/2026 12:50 PM, Shubhang Kaushik wrote: > Hi Peter, > > On Fri, 30 Jan 2026, Peter Zijlstra wrote: > >> Two issues related to reweight_entity() were raised; poking at all that got me >> these patches. >> >> They're in queue.git/sched/core and I spend most of yesterday staring at traces >> trying to find anything wrong. So far, so good. >> >> Please test. >> >> > > I’m seeing a consistent NULL pointer dereference in pick_task_fair() when running hackbench on an Ampere Altra (80 cores arm64). This is happening after applying the complete patchset on the latest 6.19.0+ kernel with PREEMPT_DYNAMIC (full), CONFIG_SCHED_CLUSTER and NOHZ_FULL enabled. Can you confirm you using the latest changes from: git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git sched/core at commit bdba3187771c ("sched/fair: Use full weight to __calc_delta()") since this series has undergone some churn throughout the week? > > The system triggers a level 2 translation fault because pick_eevdf() returns NULL despite the runqueue having active tasks (cfs_rq->nr_running >> 0). When pick_next_task_fair() attempts to dereference this NULL pointer > to access the task structure, the kernel Oopses at pick_task_fair+0x48/0x148. > > pick_task_fair <- pick_eevdf() <- [active tasks] > > The root cause is an underflow in reweight_entity(): > se->vprot -= avruntime; But how does it cause failure to pick? All vrpot does at pick is: /* curr needs to be queued and eligible first. */ if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr))) curr = NULL; /* vprot only helps for early return below. */ if (curr && protect && protect_slice(curr)) return curr; Worst case "curr" runs till it loses eligibility even if vprot wraps and becomes a large value and moreover, wouldn't vruntime_cmp() in protect_slice() which compares the signed difference catch it? -- Thanks and Regards, Prateek