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Mon, 12 Jan 2026 18:00:59 GMT Received: from smtpav07.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 12CFB2004D; Mon, 12 Jan 2026 18:00:59 +0000 (GMT) Received: from smtpav07.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 4C81B20043; Mon, 12 Jan 2026 18:00:55 +0000 (GMT) Received: from linux.ibm.com (unknown [9.124.219.206]) by smtpav07.fra02v.mail.ibm.com (Postfix) with ESMTPS; Mon, 12 Jan 2026 18:00:55 +0000 (GMT) Date: Mon, 12 Jan 2026 23:30:52 +0530 From: Vishal Chourasia To: Peter Zijlstra Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, paulmck@kernel.org, frederic@kernel.org, neeraj.upadhyay@kernel.org, joelagnelf@nvidia.com, josh@joshtriplett.org, boqun.feng@gmail.com, urezki@gmail.com, rostedt@goodmis.org, tglx@linutronix.de, sshegde@linux.ibm.com, srikar@linux.ibm.com Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during CPU hotplug operations Message-ID: References: <20260112094332.66006-2-vishalc@linux.ibm.com> <20260112142440.GN830755@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260112142440.GN830755@noisy.programming.kicks-ass.net> X-TM-AS-GCONF: 00 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTEyMDE0NyBTYWx0ZWRfX07u6aTbLgyHX QMoSyhf+ThTMv7Nb2QAJddW+K/i2OfSp/M2/d2z28wYTHBGWAFqh+QTTzCjvgA9I+S/Hj3GkV3+ qQeo+/Rqs2KqV0p92XVLVe/LbPC8bYLNH18UW4O3n7NxYnActXyqU3NYZkzMgNHh3njP/K9EhwY tWzAAqZHdWFQHMEwcs6O0y7OgvIA7sqpoAuLTA14NenVgLM9udafJG0+VPZONjaqkNJFMjEF7yq jgLQFkegt/SBS7GXKDJBvu/p6D19eAw7Ix9dfmmXyXX6rqpNNAUXDPk/27ixHPtofVdL2oKH5K4 rJtXT7Rgx1E1CD2GuKGDM7c9E/BYwAOgTZ6z+zpI17uDOszQQTTNpK1AXrOfn/DEs54MOtvFfup BOk81jOw5vYOX8LiGusd+NM7c4YkZGc7zyY7eexZ8kUy3NlTiD3OGM5/IjeopeLiIDEesucIpLQ n3JzpcXyud0NU92r/rw== X-Proofpoint-GUID: ZPJhPHiQkyhVSKs59sUcKj1E0nps_jcI X-Authority-Analysis: v=2.4 cv=WLJyn3sR c=1 sm=1 tr=0 ts=696536e0 cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=AHx039Be-gzaUm18DmAA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-ORIG-GUID: YRsmH3eoi7g55Frp2_RBWhVeRPOk4-iY X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2026-01-12_05,2026-01-09_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 adultscore=0 malwarescore=0 phishscore=0 suspectscore=0 priorityscore=1501 bulkscore=0 clxscore=1015 impostorscore=0 lowpriorityscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2512120000 definitions=main-2601120147 Hello Peter, On Mon, Jan 12, 2026 at 03:24:40PM +0100, Peter Zijlstra wrote: > On Mon, Jan 12, 2026 at 03:13:33PM +0530, Vishal Chourasia wrote: > > Bulk CPU hotplug operations—such as switching SMT modes across all > > cores—require hotplugging multiple CPUs in rapid succession. On large > > systems, this process takes significant time, increasing as the number > > of CPUs grows, leading to substantial delays on high-core-count > > machines. Analysis [1] reveals that the majority of this time is spent > > waiting for synchronize_rcu(). > > > > Expedite synchronize_rcu() during the hotplug path to accelerate the > > operation. Since CPU hotplug is a user-initiated administrative task, > > it should complete as quickly as possible. > > > > Performance data on a PPC64 system with 400 CPUs: > > > > + ppc64_cpu --smt=1 (SMT8 to SMT1) > > Before: real 1m14.792s > > After: real 0m03.205s # ~23x improvement > > > > + ppc64_cpu --smt=8 (SMT1 to SMT8) > > Before: real 2m27.695s > > After: real 0m02.510s # ~58x improvement > > > > But who cares? Its not like you'd *ever* do this, right? Users dynamically adjust SMT modes to optimize performance of the workload being run. And, yes it doesn't happen too often, but when it does, on machines with (>= 1920 CPUs) it takes more than 20 mins to finish. - vishal