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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 BL02EPF0001A0F9.mail.protection.outlook.com (10.167.242.100) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9654.16 via Frontend Transport; Fri, 27 Feb 2026 16:04:48 +0000 Received: from SATLEXMB06.amd.com (10.181.40.147) by satlexmb07.amd.com (10.181.42.216) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.2.2562.17; Fri, 27 Feb 2026 10:04:48 -0600 Received: from satlexmb08.amd.com (10.181.42.217) by SATLEXMB06.amd.com (10.181.40.147) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.39; Fri, 27 Feb 2026 10:04:48 -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; Fri, 27 Feb 2026 10:04:45 -0600 Message-ID: <6be36f62-91bb-4876-8ac5-4ebc4a4a7746@amd.com> Date: Fri, 27 Feb 2026 21:34:40 +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: [RFC PATCH] futex: Dynamically allocate futex_queues depending on nr_node_ids To: Sebastian Andrzej Siewior CC: Thomas Gleixner , Ingo Molnar , , Peter Zijlstra , "Darren Hart" , Davidlohr Bueso , =?UTF-8?Q?Andr=C3=A9_Almeida?= References: <20260128101358.20954-1-kprateek.nayak@amd.com> <20260224111342.-9hafXl5@linutronix.de> <20260225073939.HLbwiSs5@linutronix.de> <99a81417-5dbf-4c29-8dac-a9ed17ee24d6@amd.com> <20260225092213.oRHJyLG5@linutronix.de> <789f0aeb-6c46-4218-8780-742fa7ff8f8d@amd.com> <20260227151518.NzmS3z7E@linutronix.de> Content-Language: en-US From: K Prateek Nayak In-Reply-To: <20260227151518.NzmS3z7E@linutronix.de> Content-Type: text/plain; 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X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1 X-MS-Exchange-AntiSpam-MessageData-0: zktJFWeO3p5EQZ69tSJloLnh6qL0mkI2CconZpB+BOFP/XWKl6KFHdKAVQCfomq06LyASMzmVey1hRNgXfdsrGgK5vFGERk1uK9esyRTsiGp0/bBGpZSvWY0Bc711wsI1SWt74sbnJkybvFOxlKYuLhsh+uURo0ORC4ycb1KI/jIL1WYz+I1Apxt2+hMewu46DBGemVJHXuaALtjjIpccYx7Y/0bCf2Ch5nX8CKNdlHdj7JXquTpNy7ipgRkBoZWo38D27Ag4k3euI5Sdi55LuWVdDWyh365WxJNvzHVjuwDLpIQH4Jp9GxB1Kg7X5Wing4UUQo+ehiu/v1sPQPDrGH9YPK1AGQeFfIS748J8S2ZDrk4tgi6kESSwisv9OyOTvlVJz/35VV+llKJhGnUp3WlSe5q/ulp7g5CYdb4bC+2AM/0UdpKsj7gnnbjAqHA X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 27 Feb 2026 16:04:48.3405 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 73659dc9-291f-4271-7c3f-08de7619eeae 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: BL02EPF0001A0F9.namprd03.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR12MB8700 Hello Sebastian, On 2/27/2026 8:45 PM, Sebastian Andrzej Siewior wrote: >>> Sounds like it. What would be sane default upper limit then? Something >>> like 1024 CPUs? 2048? Or even more than that? >> >> I feel the current default for NR_CPUS can be be retained as is just to >> be on the safer side. >> >> Turns out QEMU allows for a ridiculous amount of vCPUs per guest and I've >> found enough evidence of extremely large guests running oversubscribed >> that sometimes run distro kernels :-( > > You mean distro kernel in 8k CPUs guest? I do this kind of things for > testing but not with a distro kernel. Oh well. I bet there are interesting justifications with stuff like live migration, reliability, etc. but I haven't dared to ask! > >> I'll let you decide what is a good trade off between space saving and >> future headaches :-) > > So you are saying NODES_SHIFT=8 and NR_CPUS=4k is what should be default > given "sane" upper limits as of today? Yup those seem sane in my opinion and amounts to a decent chunk of space saved. > I did hope for SHIFT 6 & NR=2k. Not sure this is worth fighting for. We leave a little buffer for insanity ;-) -- Thanks and Regards, Prateek