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client-ip=165.204.84.17; helo=SATLEXMB04.amd.com; pr=C Received: from SATLEXMB04.amd.com (165.204.84.17) by SA2PEPF00003F66.mail.protection.outlook.com (10.167.248.41) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.20.8655.12 via Frontend Transport; Tue, 22 Apr 2025 02:54:51 +0000 Received: from [10.136.43.233] (10.180.168.240) 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; Mon, 21 Apr 2025 21:54:46 -0500 Message-ID: <610fcbfa-a8e7-483c-a112-579879628b99@amd.com> Date: Tue, 22 Apr 2025 08:24:21 +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: [RFC PATCH v2 0/7] Defer throttle when task exits to user To: Aaron Lu CC: Jan Kiszka , Florian Bezdeka , Valentin Schneider , Ben Segall , Peter Zijlstra , Josh Don , Ingo Molnar , Vincent Guittot , Xi Wang , , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Mel Gorman , Chengming Zhou , Chuyi Zhou , "Sebastian Andrzej Siewior," References: <20250409120746.635476-1-ziqianlu@bytedance.com> <20250414120407.GC3558904@bytedance> <7483d3ae-5846-4067-b9f7-390a614ba408@siemens.com> <0a06f6c4-5d69-4fd1-badd-92fd55d8f38d@amd.com> <55687bb5-7e8a-4d7d-a597-6f97087cab32@siemens.com> <5e919998-338c-4055-b58a-e4586134956c@amd.com> <20250421120648.GA3357499@bytedance> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <20250421120648.GA3357499@bytedance> Content-Type: text/plain; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Apr 2025 02:54:51.6023 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 2542db0b-7e2b-4fe5-83df-08dd81490d90 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=[SATLEXMB04.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SA2PEPF00003F66.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DS0PR12MB9346 Hello Aaron, On 4/22/2025 7:40 AM, Aaron Lu wrote: >> anon_pipe_write() >> __wake_up_common() >> ep_poll_callback() { >> read_lock_irq(&ep->lock) /* Read lock acquired here */ > I was confused by this function's name. I had thought irq is off but > then realized under PREEMPT_RT, read_lock_irq() doesn't disable irq... Yup! Most of the interrupt handlers are run by the IRQ threads on PREEMPT_RT and the ones that do run in the interrupt context have all been adapted to use non-blocking locks whose *_irq variants disables interrupts on PREEMPT_RT too. > >> __wake_up_common() >> ep_autoremove_wake_function() >> try_to_wake_up() /* Wakes up "epoll-stall" */ >> preempt_schedule() >> ... >> >> # "epoll-stall-writer" has run out of bandwidth, needs replenish to run > Luckily in this "only throttle when ret2user" model, epoll-stall-writer > does not need replenish to run again(and then unblock the others). I can confirm that throttle deferral solves this issue. I have run Jan's reproducer for a long time without seeing any hangs on your series. I hope Florian can confirm the same. -- Thanks and Regards, Prateek