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Mon, 12 Jan 2026 10:43:32 GMT Received: from smtpav05.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8E3CF20043; Mon, 12 Jan 2026 10:43:32 +0000 (GMT) Received: from smtpav05.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id EB94720040; Mon, 12 Jan 2026 10:43:27 +0000 (GMT) Received: from [9.39.17.9] (unknown [9.39.17.9]) by smtpav05.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 12 Jan 2026 10:43:27 +0000 (GMT) Message-ID: Date: Mon, 12 Jan 2026 16:13:26 +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] cpuhp: Expedite synchronize_rcu during CPU hotplug operations To: Uladzislau Rezki 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, rostedt@goodmis.org, tglx@linutronix.de, peterz@infradead.org, sshegde@linux.ibm.com, srikar@linux.ibm.com, aboorvad@linux.ibm.com References: <20260112094332.66006-2-vishalc@linux.ibm.com> Content-Language: en-US, en-IN From: Vishal Chourasia In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Authority-Analysis: v=2.4 cv=DI6CIiNb c=1 sm=1 tr=0 ts=6964d05a cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=A0Y-CKpzo18HiFePqPcA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-GUID: m0x9YtXBEyb4lRbYIAY8EgySoP54zzBL X-Proofpoint-ORIG-GUID: 1SOZL7YqZRdHro_5ephMvGGHPczMAhry X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTEyMDA4MSBTYWx0ZWRfX6tabbbvpaXsx YTOer3oLikHQRd4dT/War0IlN166qe4U1Gps29Xcv01P3G9oLySXL/oFvV/BFFzl79RN0KL5kqG zPN2ehq5JScAH2VEcV+0Gdi5VE8/tvSOaeUyQ2mD0K57h6k1qwcD6zqvHsuo2KF2k1YxvivW0Bz Y/BCgVxflVv2B/yUsmbvm826+mh19r8kF3yLOldjZbQxJI+Emlrjyx/dtQNq7GL6ZNAABT0IdPi A94FJ/QpZ8q2R4ZRpKF84Zkz99lmas8Rh+54BGg1NXd4Jl0fIJ6h7pdPgml/h61Cam2QFJa06L5 Wy/dKeDswYIi61wvir3nhRES1I2yRrAQRfia8uXiYFgnfY3bESjYcWkmiciFbLYJymG88xBa5hy FtB4g3aqQn3O8VXXU++w2oyCERKE4pKuwV06m/Usaniie7qxQteFdzuzA9H4XTjGJ+3Y6e8Fm2c UDc6dF+dQlDvwOvFAPA== 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_03,2026-01-09_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 priorityscore=1501 lowpriorityscore=0 adultscore=0 malwarescore=0 spamscore=0 suspectscore=0 phishscore=0 impostorscore=0 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2512120000 definitions=main-2601120081 Hi Uladzislau, On 12/01/26 15:38, Uladzislau Rezki wrote: > On Mon, Jan 12, 2026 at 03:13:33PM +0530, Vishal Chourasia wrote: >> 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 >> >> Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b >> >> [1] https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.camel@linux.vnet.ibm.com >> > Also you can try: echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp > to speedup regular synchronize_rcu() call. But i am not saying that it would beat > your "expedited switch" improvement. # echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp After setting, # time ppc64_cpu --smt=1; real    1m10.726s          # Run 1 real    1m12.530s          # Run 2 # time ppc64_cpu --smt=8 real    0m36.661s          # Run 1 real    0m41.401s          # Run 2 Regards, vishalc