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McKenney" , Boqun Feng , Jonathan Corbet , Madadi Vineeth Reddy , K Prateek Nayak , Steven Rostedt , Sebastian Andrzej Siewior , Arnd Bergmann , "linux-arch@vger.kernel.org" References: <20251029125514.496134233@linutronix.de> <03687B00-0194-4707-ABCB-FB3CD5340F11@oracle.com> <2eee5e37-e541-4ac7-9479-cef3e62f234d@efficios.com> <87ldkbdmbu.ffs@tglx> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <87ldkbdmbu.ffs@tglx> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: YQZPR01CA0157.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:c01:8c::6) To YT2PR01MB9175.CANPRD01.PROD.OUTLOOK.COM (2603:10b6:b01:be::5) 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: YT2PR01MB9175:EE_|YQXPR01MB6252:EE_ X-MS-Office365-Filtering-Correlation-Id: ca1bede1-027d-44ca-dfc4-08de222c8d4c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|366016|1800799024|7416014|376014; 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I've spotted something which may explain >> the watchdog panic. Here is the scenario: >> >> 1) A process is constrained to a subset of the possible CPUs, >> and has enough threads to swap from per-thread to per-cpu mm_cid >> mode. It runs happily in that per-cpu mode. >> >> 2) The number of allowed CPUs is increased for a process, thus invoking >> mm_update_cpus_allowed. This switches the mode back to per-thread, >> but delays invocation of mm_cid_work_fn to some point in the future, >> in thread context, through irq_work + schedule_work. >> >> At that point, because only __mm_update_max_cids was called by >> mm_update_cpus_allowed, the max_cids is updated, but mc->transit >> is still zero. >> >> Also, until mm_cid_fixup_cpus_to_tasks is invoked by either the >> scheduled work or near the end of sched_mm_cid_fork, or by >> sched_mm_cid_exit, we are in a state where mm_cids are still >> owned by CPUs, but we are now in per-thread mm_cid mode, which >> means that the mc->max_cids value depends on the number of threads. > > No. It stays in per CPU mode. The mode switch itself happens either in > the worker or on fork/exit whatever comes first. Ah, that's what I missed. All good then. [...] > > There was an issue in V3 with the not-initialized transit member and a > off by one in one of the transition functions. It's fixed in the git > tree, but I haven't posted it yet because I was AFK for a week. > > I did not notice the V3 issue because tests passed on a small machine, > but after I did a rebase to the tip rseq and uaccess bits, I noticed the > failure because I tested on a larger box. Good ! We'll see if this fixes the issue observed by Prakash. If not, I'm curious to validate that num_possible_cpus() is always set to its final value before _any_ mm is created. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com