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client-ip=165.204.84.17; helo=satlexmb07.amd.com; pr=C Received: from satlexmb07.amd.com (165.204.84.17) by CH2PEPF0000009F.mail.protection.outlook.com (10.167.244.21) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9456.9 via Frontend Transport; Mon, 22 Dec 2025 06:03:43 +0000 Received: from satlexmb08.amd.com (10.181.42.217) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.17; Mon, 22 Dec 2025 00:03:42 -0600 Received: from [172.31.184.125] (10.180.168.240) by satlexmb08.amd.com (10.181.42.217) with Microsoft SMTP Server id 15.2.2562.17 via Frontend Transport; Mon, 22 Dec 2025 00:03:40 -0600 Message-ID: <1db9aefe-c9b3-4e9b-b4e3-45c25497e988@amd.com> Date: Mon, 22 Dec 2025 11:33:34 +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] softirq: allow raising NET_RX_SOFTIRQ from SMP call on RT To: , Thomas Gleixner , Sebastian Andrzej Siewior , Ingo Molnar , Peter Zijlstra CC: Tetsuo Handa , Ryo Takakura , References: <20251219175848.28587-1-wen.yang@linux.dev> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <20251219175848.28587-1-wen.yang@linux.dev> Content-Type: text/plain; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 22 Dec 2025 06:03:43.2436 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 3a0b817d-bbda-4afa-3a0a-08de411fdc86 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=[satlexmb07.amd.com] X-MS-Exchange-CrossTenant-AuthSource: CH2PEPF0000009F.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY1PR12MB9582 Hello Wen, On 12/19/2025 11:28 PM, wen.yang@linux.dev wrote: > From: Wen Yang > > RPS distributes network processing load across CPUs by enqueuing > packets on a remote CPU's backlog and raising NET_RX_SOFTIRQ to > process them. This is done via an smp_call_function. I'm assuming net_rps_send_ipi() is the function responsible for IPIs and the only two callers - net_rps_action_and_irq_enable() and dev_cpu_dead() - first check for "!use_backlog_threads()" before sending the IPI which is always false in case of PREEMPT_RT. What is the path where you end up sending this IPI on PREEMPT_RT? Can you please trace the path of this problematic IPI on the latest upstream and prevent that instead? > > On PREEMPT_RT kernels, raising a softirq from an SMP call function > can increase preemption-off latencies. A WARN_ON_ONCE() in > do_softirq_post_smp_call_flush() enforces this. > > A previous commit 6675ce20046d ("softirq: Allow raising SCHED_SOFTIRQ from SMP-call-function on RT kernel") There is also a good justification in 6675ce20046d that idle load balancing periodically checks need_resched() and will bail out immediately if it detects a wakeup but the same cannot be expected of all NET_RX_SOFTIRQ handlers so ignoring the warning isn't a solution. -- Thanks and Regards, Prateek